Abstract
Reports regarding the effects of long-term organic and inorganic fertilization on the quantity and quality of soil organic carbon (SOC), particularly in Vertisols, are scarce. In this study, we combined SOC physical fractionation with 13C NMR spectroscopy technology to investigate the effect of 34 years of continuous fertilization on the SOC physical fractions and its chemical composition of 0–20 cm soil layer in a Vertisol. This study consisted of six treatments: no fertilization (control), chemical nitrogen, phosphorus and potassium fertilizers (NPK), low and high amounts of straw with chemical fertilizers (NPKLS and NPKHS), and pig or cattle manure with chemical fertilizers (NPKPM and NPKCM). Over 34 years of continuous fertilization, the SOC sequestration rate was from 0.08 Mg C ha−1 yr−1 in the control treatment to 0.66 Mg C ha−1 yr−1 in the NPKCM treatment, which was linearly related with the C input (P < 0.01). Of the five SOC physical fractions, two silt plus clay fractions (S + C_M, S + C_mM) dominated 74–92% of SOC, while three POM fractions (cPOM fPOM and iPOM) were only 8–26%. The two manure application treatments significantly increased all the SOC physical fractions except for the silt plus clay fraction within macroaggregates (S + C_M) compared with NPK treatment (P < 0.05), which was dependent on the larger amount of C input. Also, the two manure application treatments increased the levels of alkyl C and aromatic C but decreased O-alkyl C (P < 0.05), whereas the straw application (NPKLS and NPKHS) had no impact on the C functional groups (P > 0.05). Overall, the combination of animal manure with inorganic fertilization could enhance the SOC sequestration and alter its quantity and quality in Vertisols.
Similar content being viewed by others
Introduction
The enhancement of SOC sequestration through judicious agricultural management practices is a promising strategy for improving soil fertility and crop yields1,2. Fertilization, a common agricultural management practice, can enhance SOC sequestration in cropland soils because it affects the quantity and quality of SOC3,4. Although the effects of long-term fertilization on SOC have been extensively reported in many studies, the focus of these works has been predominantly on the total SOC5,6,7 and the effects of fertilization on the quantity and quality of SOC are less defined. Therefore, a better understanding of the effects of long-term fertilization on the chemical composition of SOC and its physical fractions is necessary.
Recently, advanced solid-state 13C NMR spectroscopy has come to be universally known as a powerful tool for studying the chemical composition of SOC at a molecular level8,9. The chemical composition of SOC is usually divided into four dominant C functional groups: alkyl C, O-alkyl C, aromatic C, and carbonyl C10. O-alkyl C is primarily derived from polysaccharides from fresh plant materials that readily decompose11. In contrast, both alkyl C and aromatic C mainly consist of original plant biopolymers, soil microbial metabolites and lignin and are regarded as stable organic C12. Using this advanced technology, the effects of fertilization on the chemical composition of SOC have been reported in many studies13,14,15. Ultisol amended with pig manure (42 000 kg ha−1 yr−1) demonstrated more aromatic C, O-alkyl C and carbonyl C than Ultisol amended with straw15. Wang et al.15 also reported obvious differences in the molecular characteristics of SOC, particularly in its density and aggregate fractions, after 4 years of pig-manure compost application in an Anthrosol (paddy soil) in Changshu, China. In contrast, Yan et al.4 reported that 31 years of continuous application of pig manure (15 000 kg ha−1 yr−1) had a minimal effect on the chemical composition of SOC in an Anthrosol (paddy soil) in Jinxian, China. These previous reports indicate that, the chemical composition of SOC may be controlled to some extent by the quantity and quality of the organic amendments. Thus, the impact that different quantities and qualities of organic amendments have on the chemical composition of SOC must be assessed via 13C NMR spectroscopy technology.
The total SOC is not always a sensitive indicator for detecting changes in or elucidating the mechanisms of SOC sequestration with different agricultural management practices16 because SOC is heterogeneous, dynamic and consists of different fractions that vary in their physical and chemical properties, stabilities and turnover rates17. To better characterize, predict, and potentially manage SOC sequestration, the total SOC can be generally separated into five fractions by the physical fractionation technique: coarse particulate organic matter (cPOM), fine inter-microaggregate particulate organic matter (fPOM), intra-microaggregate particulate organic matter within macroaggregates (iPOM), silt plus clay fraction within macroaggregate (S + C_M), and silt plus clay fraction within microaggregates occluded within macroaggregates (S + C_mM)18. The three POM fractions are a mixture of compounds comprised mainly of plant residues and partial microbial decompositions together, whilst S + C_mM and S + C_M fractions are mineral-associated C (silt and clay protected C). These SOC fractions vary in their sensitivity and responsiveness to the changes induced by fertilization practices. For example, long-term chemical fertilization alone generally had no effect19,20,21 or a positive effect22 on SOC fractions compared with unfertilized soil. However, He et al.22 reported that a combination of straw and chemical fertilization increased the content of cPOM, iPOM, and S + C fractions in an Inceptisol (in Zhengzhou) but decreased in a Mollisol (in Gongzhuling). In contrast, manure application significantly increased all SOC fractions in an Ultisol23, Anthrosol24 and Inceptisol22. Clearly, the effect of various fertilization practices on the SOC physical fractions remains largely unknown.
Vertisols (locally referred to as Shajiang black soil) cover an area of approximately 4 × 106 ha of the Huang-Huai-Hai Plain of China, which is one of the most important wheat production areas in the country. The low SOC content of Vertisols is a major factor for limiting crop yields according to our large-scale survey (data unpublished). To enhance SOC sequestration in Vertisols, the application of straw and manure is encouraged by local policy makers. The long-term application of these amendments plays a critical role in the chemical composition of SOC, the level of each fraction, and subsequently crop yields. We hypothesized that the impact of organic and inorganic fertilization on the SOC sequestration rate and chemical composition would vary, according to differences in the quantity and quality of the amendments used. Therefore, the objectives of this study were to (1) evaluate the long-term effects of fertilization with straw and manure on crop yields and SOC sequestration rates, (2) determine the effect of long-term fertilization on the chemical composition of SOC with 13C NMR spectroscopy technology, and (3) determine the response of SOC fractions to different fertilization practices with the SOC physical fractionation technique.
Materials and Methods
Site description
The research site is located at the Madian Agro-Ecological station, Anhui Province, in the Huang-Huai Plain of China. This region has a typical sub-humid climate: the average annual temperature is 16.5 °C, and the average annual rainfall is 872 mm. The soil is locally referred to as Shajiang black soil and classified as Vertisol according to the USDA soil taxonomy25, and montmorillonite is the dominant clay mineral. This long-term fertilization experiment was initiated in 1982. Prior to the establishment of the experiment (1982), the initial soil in the plough layer (0–20 cm) contained 5.8 g kg−1 organic Carbon (C), 0.96 g kg−1 total Nitrogen (N), 0.28 g kg−1 total Phosphorus (P), 280 g kg−1 sand, 306 g kg−1 silt, and 414 g kg−1 clay, and it had a pH (1:2.5 soil/water) of 7.4 and a bulk density of 1.45 g kg−1.
Experimental design
This long-term fertilization experiment consisted of six treatments with a randomized complete block design. Each treatment had four replicates and each plot was 75 m2 (15 m × 5 m). The six treatments were as follows: (1) control, no fertilization, (2) NPK, chemical nitrogen, phosphorus and potassium fertilizers, (3) NPKLS, NPK fertilizers plus 3 750 kg ha−1 yr−1 wheat straw, (4) NPKHS, NPK fertilizers plus 7 500 kg ha−1 yr−1 wheat straw, (5) NPKPM, NPK fertilizers plus 15 000 kg ha−1 yr−1 fresh pig manure, and (6) NPKCM, NPK fertilizers plus 30 000 kg ha−1 yr−1 fresh cattle manure. The moisture contents of the straw, pig manure and cattle manure were 33.3%, 48% and 58.3%, respectively. The doses of N, P2O5 and K2O applied to the Vertisol were 180 kg ha−1, 90 kg ha−1 and 135 kg ha−1, respectively. All inorganic and organic fertilizers used as basal fertilizers were applied before the sowing of the wheat in October. The crop system was wheat-soybean rotation.
Crop yield and soil sampling
Wheat or soybean yield was obtained after harvesting the crops of each plot and converted to 14% moisture content for weight calculation. The data of crop yield were from 2012 to 2016. After the soybean harvest in early October, 2016, soil samples were collected from five randomly selected sites of each plot at a depth of 0–20 cm to form a composite sample. Visible pieces of crop debris and roots were removed from the soil sample. The soil samples were air dried, ground to pass through a 2 mm sieve and stored at room temperature for SOC NMR spectroscopy and physical fraction analysis.
NMR spectroscopy analysis
The chemical composition of SOC was measured with solid-state 13C nuclear magnetic resonance (NMR) spectroscopy technology according to the method described in detail by Gonçalves et al.26. Prior to the NMR analysis, soil samples were pretreated with a 10% hydrofluoric acid (HF) solution to remove paramagnetic components, concentrate their relative C content and increase the spectral quality. Briefly, 5 g of air-dried soil (<2 mm) was transferred into a 100 ml polyethylene tube. After the addition of 40 ml of 10% (w/w) HF, the tubes were closed and then vigorously shaken for 30 s. All tubes were subsequently placed in an incubator shaker for 2 h. After shaking, they were centrifuged at 3000 rpm for 10 min at room temperature. The supernatant was decanted and discarded. The residue was again washed with an equal volume of HF. This procedure was repeated at least 12 times. The remaining soil material was washed four times with distilled water, transferred to a 25 ml polyethylene tube, and then freeze dried. The samples were finally ground to pass through a 0.15 mm plastic sieve for NMR spectroscopy analysis.
The CPMAS-13C NMR spectroscopy was performed with a Bruker Avance III 400 MHz NMR spectrometer operating at 100.4 MHz. The entire chemical shift region of the 13C NMR spectroscopy for each treatment is shown in Fig. 1. According to the chemical shift regions and the spectroscopy assignments described by Kögel-Knabner (1997)27, the 13C NMR spectral was generally divided into the following C functional groups: (1) alkyl C (0–45 ppm) - terminal methyl groups, methylene groups in aliphatic rings and chains, (2) methoxyl C (45–60 ppm) - methoxyl groups, also classified as O-alkyl C (C-6 for carbohydrates and sugars, C-a for most amino acids), (3) carbohydrate C (60–90 ppm) - carbohydrate-derived structures (C-2 to C-5) in hexoses and higher alcohols (C-a for some amino acids), (4) di-O-alkyl C (90–110 ppm) - anomeric carbon of carbohydrates (C-2, C-6 and syringyl units of lignin), (5) aryl C (110–142 ppm) - aromatic C-H carbons (guaiacyl, C-2, C-6 in lignin, and olefinic carbons), (6) phenolic C (142–160 ppm) - aromatic COR or CNR groups, and (7) carboxyl C (160–220 ppm) carboxyl/carbonyl/amide carbons, in which (2), (3) and (4) can be combined as O-alkyl C (45–110 ppm), (5) and (6) as aromatic C (110–160 ppm). The ratio of alkyl C/O-alkyl C was used as an index to assess the degree of SOM decomposition28. The aromaticity was used as an index to characterize the extent of humification of the SOM29, which was calculated with the following equation:
SOC physical fractions
The method used for SOC physical fractionation was adopted from Six et al.12. The bulk soil was sorted into cPOM, mM, S + C_M by using the wet sieving method. Briefly, 50 g of <2 mm air-dried soil and 50 glass beads (diameter = 4 mm) were placed on a 250 μm sieve. Before wet sieving, each soil sample was soaked in deionized water for 10 min. After removing any floating litter, the sieve was manually agitated 50 times over 2 min (approximately 25 3-cm oscillations min−1). After removing the beads form the sieve, any material that was >250 μm (cPOM plus sand) was left on the sieve. All <250 μm materials were then flushed immediately onto a 53 μm sieve with a continuous and steady water flow. The soil remaining on the 53 μm sieve was sieved manually in the same way, to isolate the mM (53–250 μm) and the S + C_M (<53 μm) and transferred in its entirety into an aluminum box.
Next, the 53–250 μm heavy fraction and fPOM were separated by density flotation. Briefly, a 5 g subsample of the microaggregate was oven dried (110 °C) overnight and suspended in 35 ml of 1.85 g cm−3 sodium iodide (NaI) in a 100 ml centrifuge tube. The suspension was shaken reciprocally by hand for 30 strokes, and the material remaining on the cap and sides of the centrifuge tube was washed into the suspension twice with 5 ml of sodium iodide. The sample was then put in a vacuum chamber for 10 min. After 20 min of equilibration, the sample was centrifuged at 3500 r min−1 for 10 min, and then the supernatant was immediately filtered through a 0.45 μm filter membrane under vacuum and the NaI was collected for reuse. The materials retained on the filter membrane (defined as fPOM) were rinsed into an aluminum box with deionized water at least three times. The heavy fraction remaining in the centrifuge tube was rinsed twice with deionized water, and dispersed in 50 ml of 0.5% sodium hexametaphosphate (HMP) by shaking at 300 r min−1 for 18 h on a reciprocal shaker. Finally, the dispersed heavy fraction was passed through a 53 μm sieve to isolate the iPOM (53–250 μm) and S + C_mM (<53 μm). All fractions were dried at 50 °C and weighed. The C contents of various fractions were measured with an elemental analyser (Vario MAX CN, Germany).
Estimation of the C input and SOC sequestration rate
The C input (Cinput) (Mg ha−1 yr−1) was estimated according to the stubble and root derived C (Croot+stubble), straw-returned C (Cstraw) and manure-applied C (Cmanure) with the following equations:
where Ygrain and Ystraw are the grain and straw yields (kg ha−1), respectively; 1.1 and 1.6 are the ratios of straw to grain for wheat and soybean, respectively30; R is the ratio of root biomass to total aboveground biomass (0.429 for wheat and 0.235 for soybean); Rroot is the ratio of the root system within the topsoil (0–20 cm) (0.753 for wheat and 0.984 for soybean); Rstubble is the coefficient of stubble (0.13 for wheat and 0.15 for soybean); W is the water content of air-dried gain (14%); OCcrop is the C content of air-dried crop (399 g kg−1 for wheat and 453 g kg−1 for soybean). Bstraw and Bmaure are the straw biomass and manure biomass (kg ha−1), respectively; OCstraw, and OCmanure are the C contents of wheat straw (482 g kg−1), and manure (366 g kg−1 for pig manure and 374 g kg−1 for cattle manure), respectively6. The SOC sequestration rate (Mg ha−1 yr−1) was calculated with the following equation:
where SOCcurrent and SOCinitial are the content of SOC in 2016 and the initial year (1982), respectively; ρ is the soil bulk density (g cm−3); H is the depth of the soil layer (20 cm); and T is the period of the experiment (34 years). The SOC sequestration efficiency (%) was calculated as follows:
where C input is annual C input via stubble and root, straw and manure.
Statistical analysis
The data analysis was performed with SPSS 22.0 software for Windows (SPSS Inc., USA). The difference in yield, C inputs, and SOC sequestration rate, chemical composition of the SOC and its fractions among the fertilization treatments were assessed with a one-way analysis of variance (ANOVA) and a least significant difference (LSD) test. A simple linear-regression analysis was conducted to reveal the relationships between SOC sequestration and crop yields or C inputs. All analyses were considered significant at P < 0.05.
Results
Crop yields
The annual wheat and soybean yields with different fertilization treatments from 2012 to 2016 are shown in Fig. 2. The two animal manures combined with NPK fertilization (NPKPM and NPKCM) increased the annual wheat yield by 8.17% and 11.3% relative to the NPK treatment (P < 0.05), respectively. The highest increase in the annual soybean yield was also observed in these two treatments, which presented yields of 43.0% and 77.2%, respectively. These values are appreciably larger than those for the NPK treatment (P < 0.05). The high amount of straw return (NPKHS) also significantly increased the annual wheat and soybean yields (8.27% and 19.4%, respectively) compared with the effects of NPK fertilization (P < 0.05).
C input and SOC sequestration
The estimated C inputs and SOC sequestration rates in the plow layer (0–20 cm) for each treatment after 34 years of continuous fertilization are listed in Table 1. The C input in the combined straw or manure treatments with inorganic fertilization (3.24–7.45 Mg C ha−1 yr−1) was much greater than that in the NPK (2.16 Mg C ha−1 yr−1) and control (0.23 Mg C ha−1 yr−1) treatments. The C input in the form of stubble and root for wheat (0.13–1.76 Mg C ha−1 yr−1) was greater than that of soybean (0.04–0.88 Mg C ha−1 yr−1) for a given fertilization practice. Consequently, compared to the control treatment, the NPK application increased the SOC content by 1.22 g kg−1. Compared with the NPK treatment, the long-term inorganic and organic combined fertilization treatment increased the SOC content by 16–132%, and these values are almost equivalent to the increase in the C storage of 3.02–25.02 Mg C ha−1 when relative to the initial SOC content level in 1982 (5.86 g kg−1). The long-term inorganic and organic fertilization treatments increased the SOC sequestration rate considerably relative to the unfertilized control treatment (Table 1) (P < 0.05). The smallest increase was observed in the NPK treatment (0.16 Mg C ha−1 yr−1), while the largest increase was observed in the NPKPM (0.57 Mg C ha−1 yr−1) and NPKCM (0.66 Mg C ha−1 yr−1) treatments. A linear relationship between the C input and SOC sequestration rate is shown in Fig. 3 (R2 = 0.92, P < 0.01). The highest SOC sequestration efficiency was observed in the control treatment (33.4%). The SOC sequestration efficiencies for the inorganic and organic fertilization (NPKLS, NPKHS, NPKPM and NPKCM) varied from 7.12% to 10.7%, but a significant difference was not observed among the fertilization treatments (P > 0.05) (Table 1).
SOC chemical composition
The C functional groups under fertilization, are shown in Table 2. The 13C NMR spectroscopy results showed that O-alkyl C (50.9–55.2%) predominated, followed by alkyl C (22.5–26.1%), aromatic C (12.2–14.3%) and carbonyl C (8.6–10.0%). An increase in the proportion of alkyl C and a decrease in that of O-alkyl C in the NPKPM and NPKCM treatments were observed compared with the levels in the NPK treatment (P < 0.05). The ratio of alkyl C/O-alkyl C and the aromaticity were all greater in the NPKPM and NPKCM treatments than in the other fertilization treatments.
SOC physical fractions
The SOC fractions were greatly altered by long-term fertilization (Table 3). The total SOC recovery from the bulk soil after wet sieving, density flotation and dispersion was 90% to 98%. Of the five SOC fractions, the S + C_mM fraction predominated at 42–50%, followed by the S + C_M fraction (20–42%) and then the iPOM fraction (4–12%), fPOM fraction (2–10%) and cPOM fraction (2–4%). The inorganic fertilization treatment (NPK) significantly increased the content of all SOC fractions except for the cPOM fraction compared with the unfertilized treatment (control) (P < 0.05). The combined organic and inorganic fertilization treatment (NPKHS, NPKPM and NPKCM) notably increased the content of the cPOM (57–238%), fPOM (77–313%), iPOM (74–319%) and S + C_mM (32–130%) fractions (P < 0.05) relative to that of the NPK treatment but did not increase the content of the S + C_M fraction (P > 0.05). Carbon inputs were linearly related to the cPOM (R2 = 0.88, P < 0.01), fPOM (R2 = 0.94, P < 0.01), iPOM (R2 = 0.89, P < 0.01) and S + C_mM (R2 = 0.89, P < 0.01).
Discussion
Crop yield in the Vertisol under long-term fertilization
The annual yield of both wheat and soybean increased after long-term fertilization compared with no fertilization (P < 0.05). In particular, the greatest increase in yield for both wheat and soybean was observed in the animal manure application treatments (NPKPM and NPKCM). Similar findings have been observed in previous studies31,32. The positive effects of manure application on crop yield are likely due to the improved physical environment of the soil coupled with the increased availability of the nutrients necessary for crop growth33,34. Increases in crop yield with a simultaneous increase in SOC have been reported in several cropping systems by numerous researchers35,36. In this study, regression analysis showed that the mean increases in wheat (R2 = 0.77, P < 0.05) and soybean (R2 = 0.94, P < 0.01) yields were linearly correlated with increased SOC sequestration, which indicated that the SOC is one of the most important factors of crop production in a Vertisol. Although the soybean crop did not receive any fertilization after the wheat harvest, the soybean yield showed a more obvious increase than the wheat yield did, compared to the effects of inorganic fertilizer alone treatments following 34 continuous years of fertilization in this study. This finding suggests that obtaining high yields in a rainfed wheat-soybean system depends not only on nutrients but also on other factors including plant water availability. In fact, approximately two-thirds of the annual rain fall occurs from June to September during the soybean growing season, while only one-third falls from October to May, during the winter wheat growing season37. Thus, both appropriate fertilization practices and better environment conditions need to be seriously addressed for the development of sustainable agriculture in a Vertisol.
Relationship between the C input and SOC sequestration in the Vertisol
The amount of C sequestered in the Vertisol increased considerably (16–132%) after 34 years of continuous organic fertilization compared with inorganic fertilization, which is consistent with several previous studies4,20,21. Long-term fertilization increased the SOC sequestration in at least two distinct ways. Firstly, straw and manure themselves act as an exogenous C source contributing to SOC sequestration, and secondly, increasingly more balanced fertilization may result in better plant growth, which may in turn result in a more pronounced rooting system. Increased plant growth due to balanced plant nutrition may also result in higher amounts of crop residues that are returned to the soil after harvest. Interestingly, the unfertilized control treatment also increased SOC content by 28% when compared to the initial condition. This finding is consistent with the results reported by Bhattacharyya et al.38, who showed that the cultivation of wheat and soybean for 30 years in an Inceptisol in the Indian Himalayas without any added organic and/or inorganic fertilizers increased the SOC content by 9%. The C from wheat crop residues in the unfertilized control treatment was estimated at 140 kg ka−1 yr−1 and that from soybean residues was estimated at 90 kg ha−1 yr−1, which may be enough to mitigate C losses via SOM decomposition39.
In most cases, the relationship between C inputs and C sequestration is best fit with a logarithmic equation, particularly when the C input range is wide, while a linear relationship can occur with a narrow range of C inputs40. Our result (0.16–7.39 Mg C ha−1 yr−1) showed a linear relationship (Fig. 3), as did the results reported by Zhang et al.41 across six sites in China (0.81–11.1 Mg C ha−1 yr−1). This linear relationship implies a continuous increase of SOC stock with increasing C inputs40,42. Thus, Vertisols still have great potential for C sequestration. Compared with the NPK treatment, the combined animal manure and inorganic fertilization treatments increased the SOC sequestration by 19.6–43.8%. Our results are consistent with a number of studies based on long-term fertilization experiments20,21 but inconsistent with others23,43 in which the SOC sequestration efficiency under combined organic and inorganic fertilization treatments was almost half that in the inorganic fertilization alone treatments. This is because the mechanisms controlling SOC sequestration efficiency are complex and are affected not only by soil properties44 but also by the quantity and quality of the C input, C stabilization and the saturation deficit6,45.
Chemical composition of SOC in the Vertisol
Differences in the proportions of C functional groups between unfertilized and fertilized soils may be strongly related to the amount of C inputs46. However, our results suggest that the quality of C input might directly influence the C functional group proportions. Compared with the control or NPK treatment, we found that the animal manure application treatments (NPKPM and NPKCM) increased alkyl C and aromatic C but decreased O-alkyl C, while the straw application treatments (NPKLS and NPKHS) had no impact on the C functional groups. Both animal manures were composted before application so that any readily decomposed C was lost, which resulted in increase in recalcitrant C (i.e., alkyl C, aromatic C) and decrease in easily decomposed C (i.e., O-alkyl C)47,48,49. Similar results have been reported in an Anthrosol by Zhou et al.50, who observed the negative correlation of recalcitrant C with easily decomposed C after the long-term application of animal manures. These results indicate that after animal manure application, recalcitrant C is preferentially preserved during the process of SOC sequestration relative to other C functional groups, which leads to improved SOC stability. Thus, the enhancement of SOC sequestration though animal manure application to a Vertisol shifts the accumulation of C in favour of recalcitrant C. In addition, we also found that the ratio of alkyl C/O-alkyl C was higher in the animal manure application treatments than the other fertilization treatments, which is consistent with the study by Wang et al.15 in a rice-wheat cropping system. The higher ratio of alkyl C/O-alkyl C after animal manure application suggests that this treatment may accelerate the decomposition of the O-alkyl C contained in manures into soil, which results in soil being unable to effectively accumulate labile organic C51. Composting animal manure before application is therefore a necessary part of cropland management52. From the perspective of C sequestration, reducing the loss of easily decomposed C during the composting process is desirable.
Sequestration mechanisms of SOC fractions in the Vertisol
SOC sequestration is jointly controlled by three principal mechanisms: (i) the molecular recalcitrance of OM, (ii) the physical protection of SOC, and (iii) the biochemical protection of SOC53. The proportions of the cPOM and fPOM fractions varied from 4% to 14% of the total SOC in this study, which correspond reasonably well with Christensen54, who stated that these two fractions generally accounted for less than 10% of the total SOC in the plough layer in cropland soils. The highest increase in the content of cPOM and fPOM fractions was observed in the NPKPM (161% and 238%, respectively) and NPKCM (152% and 313%, respectively) treatments (Table 3). This may be due to the recalcitrance of the manure under higher amounts of C input55. The decomposed manure contained more recalcitrant C at a molecular level as mentioned above. In addition, long-term manure application considerably increased the content of the iPOM (138% and 319%, respectively) and S + C_mM (88% and 130%, respectively) fractions via physical and biochemical protection, thereby further increasing the long-term C sequestration in a Vertisol. Our results are consistent with some previous works20,56, which is likely due to the ability of manure application to promote the formation of microaggregates within macroaggregates57,58.
Carbon inputs are the dominant driver for sequestering C in soil39. Four SOC fractions (cPOM, fPOM iPOM and S + C_mM) did exhibited a linear relationship with the C inputs in this study (Fig. 4), which indicates that these SOC fractions still have a substantial C saturation deficit, and could still stabilize the additional amounts of C inputs, thereby continuing to act as atmospheric C sinks59,60. Among these four fractions, the S + C_mM fraction was the most sensitive to fertilization practices and hence could be used as a diagnostic fraction for future management-induced changes of SOC in Vertisols. Furthermore, the differences in the responsivity of each SOC fractions also indicates that shifting the SOC towards relatively more labile organic C by increasing C inputs to Vertisols is a viable long-term fertilization management practice.
Conclusions
Based on thirty-four years of the continuous application either of straw or manure in a Vertisol, the straw return management enhanced the SOC sequestration but did not alter the SOC quality relative to the unfertilized control treatment. However, application of pig or cattle manure increased SOC storage as well as the contents of alkyl C and aromatic C due to the high quality of the C input. Relative to the straw amendments, manure application significantly increased the SOC physical fractions, particularly in the POM and aggregate associated with the silt and clay C fraction through greater C input. As a result, the SOC sequestration efficiency in the straw amendment treatments was the same as that in the inorganic fertilization alone treatment but lower than that in the manure amendment treatments. Our results indicated that a balanced application of NPK fertilizers with manure should be encouraged to improve SOC sequestration and ensure greater agricultural production in Vertisols.
References
Lal, R. Soil Carbon Sequestration Impacts on Global Climate Change and Food Security. Science 304, 1623–1627 (2004).
Pan, G. X. et al. Combined inorganic/organic fertilization enhances N efficiency and increases rice productivity through organic carbon accumulation in a rice paddy from the Tai Lake region, China. Agric. Ecosyst. Environ. 131, 274–280 (2009).
Marriott, E. E. & Wander, M. M. Total and Labile Soil Organic Matter in Organic and Conventional Farming Systems. Soil Sci. Soc. Am. J. 70, 950–959 (2006).
Yan, X. et al. Carbon sequestration efficiency in paddy soil and upland soil under long-term fertilization in southern China. Soil Till. Res. 130, 42–51 (2013).
Cai, Z. C. & Qin, S. W. Dynamics of crop yields and soil organic carbon in a long-term fertilization experiment in the Huang-Huai-Hai Plain of China. Geoderma 136, 708–715 (2006).
Hua, K. K., Wang, D. Z., Guo, X. S. & Guo, Z. B. Carbon Sequestration Efficiency of Organic Amendments in a Long-Term Experiment on a Vertisol in Huang-Huai-Hai Plain, China. PLOS ONE 9, e108594 (2014).
Li, Z. P., Liu, M., Wu, X. C., Han, F. X. & Zhang, T. L. Effects of long-term chemical fertilization and organic amendments on dynamics of soil organic C and total N in paddy soil derived from barren land in subtropical China. Soil Till. Res. 106, 268–274 (2010).
Mao, J. D., Olk, D. C., Fang, X. W., He, Z. Q. & Schmidt-Rohr, K. Influence of animal manure application on the chemical structures of soil organic matter as investigated by advanced solid-state NMR and FT-IR spectroscopy. Geoderma 146, 353–362 (2008).
Song, X. Y. et al. Differences of C sequestration in functional groups of soil humic acid under long term application of manure and chemical fertilizers in North China. Soil Till. Res. 176, 51–56 (2018).
Helfrich, M., Ludwig, B., Buurman, P. & Flessa, H. Effect of land use on the composition of soil organic matter in density and aggregate fractions as revealed by solid-state 13C NMR spectroscopy. Geoderma 136, 331–341 (2006).
Krull, E. S., Baldock, J. A. & Skjemstad, J. O. Importance of mechanisms and processes of the stabilisation of soil organic matter for modelling carbon turnover. Funct. Plant Biol. 30, 207–222 (2003).
Kögel-Knabner, I., de Leeuw, J. W. & Hatcher, P. G. Nature and distribution of alkyl carbon in forest soil profiles: implications for the origin and humification of aliphatic biomacromolecules. Sci. Total Environ. 117–118, 175–185 (1992).
Li, Z. Q., Zhao, B. Z., Wang, Q. Y., Cao, X. Y. & Zhang, J. B. Differences in Chemical Composition of Soil Organic Carbon Resulting From Long-Term Fertilization Strategies. PloS one 10, e0124359 (2015).
Zhang, J. C. et al. The role of non-crystalline Fe in the increase of SOC after long-term organic manure application to the red soil of southern China. Eur. J. Soil Sci. 64, 797–804 (2013).
Wang, Q. J. et al. Effects of compost on the chemical composition of SOM in density and aggregate fractions from rice–wheat cropping systems as shown by solid-state 13C-NMR spectroscopy. J. Plant Nutr. Soil Sci. 175, 920–930 (2012).
Six, J. et al. Measuring and Understanding Carbon Storage in Afforested Soils by Physical Fractionation. Soil Sci. Soc. Am. J. 66, 1981–1987 (2002).
Del Galdo, I., Six, J., Peressotti, A. & Francesca Cotrufo, M. Assessing the impact of land-use change on soil C sequestration in agricultural soils by means of organic matter fractionation and stable C isotopes. Global Change Biol. 9, 1204–1213 (2003).
Six, J., Conant, R. T., Paul, E. A. & Paustian, K. Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils. Plant Soil 241, 155–176 (2002).
Brown, K. H. et al. A long-term nitrogen fertilizer gradient has little effect on soil organic matter in a high-intensity maize production system. Glob. Change Biol. 20, 1339–1350 (2014).
Tian, J. et al. Response of soil organic matter fractions and composition of microbial community to long-term organic and mineral fertilization. Biol. Fertil. Soils 53, 523–532 (2017).
Xu, X. R. et al. Characteristics of differently stabilised soil organic carbon fractions in relation to long-term fertilisation in Brown Earth of Northeast China. Sci. Total Environ. 572, 1101–1110 (2016).
He, Y. T. et al. Long-term combined chemical and manure fertilizations increase soil organic carbon and total nitrogen in aggregate fractions at three typical cropland soils in China. Sci. Total Environ. 532, 635–644 (2015).
Tong, X. G. et al. Long-term fertilization effects on organic carbon fractions in a red soil of China. Catena 113, 251–259 (2014).
Xie, J. et al. Responses of crop productivity and physical protection of organic carbon by macroaggregates to long-term fertilization of an Anthrosol. Eur. J. Soil Sci. 69, 555–567 (2018).
Soil Survey Staff. Keys to Soil Taxonomy, 11th ed. (USDA Natural Resources Conservation Service, Washington, DC, 2010).
Gonçalves, C. N. et al. The effect of 10% HF treatment on the resolution of CPMAS 13C NMR spectra and on the quality of organic matter in Ferralsols. Geoderma 116, 373–392 (2003).
Kögel-Knabner, I. 13C and 15N NMR spectroscopy as a tool in soil organic matter studies. Geoderma 80, 243–270 (1997).
Baldock, J. A. et al. Assessing the extent of decomposition of natural organic materials using solid-state 13C NMR spectroscopy. Soil Res. 35, 1061–1084 (1997).
Mathers, N. J. & Xu, Z. Solid-state 13C NMR spectroscopy: characterization of soil organic matter under two contrasting residue management regimes in a 2-year-old pine plantation of subtropical Australia. Geoderma 114, 19–31 (2003).
NCATS. Chinese organic fertilizer handbook. National Center for Agricultural Technology Service, Chinese Agricultural Publisher. (1999) (In Chinese)
Hati, K. M., Swarup, A., Dwivedi, A. K., Misra, A. K. & Bandyopadhyay, K. K. Changes in soil physical properties and organic carbon status at the topsoil horizon of a vertisol of central India after 28 years of continuous cropping, fertilization and manuring. Agric. Ecosyst. Environ. 119, 127–134 (2007).
Zhang, X. et al. Effects of enhancing soil organic carbon sequestration in the topsoil by fertilization on crop productivity and stability: Evidence from long-term experiments with wheat-maize cropping systems in China. Sci. Total Environ. 562, 247–259 (2016).
Edmeades, D. C. The long-term effects of manures and fertilisers on soil productivity and quality: a review. Nutr. Cycl. Agroecosyst. 66, 165–180 (2003).
Rasool, R., Kukal, S. S. & Hira, G. S. Soil organic carbon and physical properties as affected by long-term application of FYM and inorganic fertilizers in maize–wheat system. Soil Till. Res. 101, 31–36 (2008).
Singh Brar, B., Singh, J., Singh, G. & Kaur, G. Effects of Long Term Application of Inorganic and Organic Fertilizers on Soil Organic Carbon and Physical Properties in Maize–Wheat Rotation. Agronomy 5, 220–238 (2015).
Yang, Z. C., Zhao, N., Huang, F. & Lv, Y. Z. Long-term effects of different organic and inorganic fertilizer treatments on soil organic carbon sequestration and crop yields on the North China Plain. Soil Till. Res. 146, 47–52 (2015).
Qin, W., Wang, D. Z., Guo, X. S., Yang, T. M. & Oenema, O. Productivity and sustainability of rainfed wheat-soybean system in the North China Plain: results from a long-term experiment and crop modelling. Sci. Rep. 5, 17514 (2015).
Bhattacharyya, R., Kundu, S., Prakash, V. & Gupta, H. S. Sustainability under combined application of mineral and organic fertilizers in a rainfed soybean–wheat system of the Indian Himalayas. Eur. J. Agron. 28, 33–46 (2008).
Kundu, S., Bhattacharyya, R., Prakash, V., Ghosh, B. N. & Gupta, H. S. Carbon sequestration and relationship between carbon addition and storage under rainfed soybean–wheat rotation in a sandy loam soil of the Indian Himalayas. Soil Till. Res. 92, 87–95 (2007).
Stewart, C. E., Paustian, K., Conant, R. T., Plante, A. F. & Six, J. Soil carbon saturation: concept, evidence and evaluation. Biogeochemistry 86, 19–31 (2007).
Zhang, W. J. et al. Soil organic carbon dynamics under long-term fertilizations in arable land of northern China. Biogeosciences 7, 409–425 (2010).
Majumder, B. et al. Organic Amendments Influence Soil Organic Carbon Pools and Rice–Wheat Productivity. Soil Sci. Soc. Am. J. 72, 775–785 (2008).
Purakayastha, T. J., Rudrappa, L., Singh, D., Swarup, A. & Bhadraray, S. Long-term impact of fertilizers on soil organic carbon pools and sequestration rates in maize–wheat–cowpea cropping system. Geoderma 144, 370–378 (2008).
Liang, F. et al. Three-decade long fertilization induced soil organic carbon sequestration depends on edaphic characteristics in six typical croplands. Sci. Rep. 6, 30350 (2016).
Zhao, Y. N., Zhang, Y. Q., Liu, X. Q., He, X. H. & Shi, X. J. Carbon sequestration dynamic, trend and efficiency as affected by 22-year fertilization under a rice–wheat cropping system. J. Plant Nutr. Soil Sci. 179, 652–660 (2016).
He, Y. T. et al. Long-term fertilization increases soil organic carbon and alters its chemical composition in three wheat-maize cropping sites across central and south China. Soil Till. Res. 177, 79–87 (2018).
Hua, K. K., Wang, D. Z. & Guo, Z. B. Soil organic carbon contents as a result of various organic amendments to a vertisol. Nutr. Cycl. Agroecosyst. 108, 135–148 (2017).
Ussiri, D. A. N. & Johnson, C. E. Characterization of organic matter in a northern hardwood forest soil by 13C NMR spectroscopy and chemical methods. Geoderma 111, 123–149 (2003).
Tang, J. C., Maie, N., Tada, Y. & Katayama, A. Characterization of the maturing process of cattle manure compost. Process Biochem. 41, 380–389 (2006).
Zhou, P., Pan, G. X., Spaccini, R. & Piccolo, A. Molecular changes in particulate organic matter (POM) in a typical Chinese paddy soil under different long-term fertilizer treatments. Eur. J. Soil Sci. 61, 231–242 (2010).
Caricasole, P., Provenzano, M. R., Hatcher, P. G. & Senesi, N. Evolution of organic matter during composting of different organic wastes assessed by CPMAS 13C NMR spectroscopy. Waste Manage. 31, 411–415 (2011).
Bernal, M. P., Alburquerque, J. A. & Moral, R. Composting of animal manures and chemical criteria for compost maturity assessment. A review. Bioresour. Technol. 100, 5444–5453 (2009).
Lützow, M. V. et al. Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review. Eur. J. Soil Sci. 57, 426–445 (2006).
Christensen, B. T. Physical fractionation of soil and structural and functional complexity in organic matter turnover. Eur. J. Soil Sci. 52, 345–353 (2001).
Sleutel, S., De Neve, S., Németh, T., Tóth, T. & Hofman, G. Effect of manure and fertilizer application on the distribution of organic carbon in different soil fractions in long-term field experiments. Eur. J. Agron. 25, 280–288 (2006).
Jiang, M. B. et al. Variation of soil aggregation and intra-aggregate carbon by long-term fertilization with aggregate formation in a grey desert soil. Catena 149, 437–445 (2017).
Huang, S., Peng, X. X., Huang, Q. R. & Zhang, W. J. Soil aggregation and organic carbon fractions affected by long-term fertilization in a red soil of subtropical China. Geoderma 154, 364–369 (2010).
Yuan, Y., Li, L., Li, N., Yang, C. & Han, X. Long-term fertilization effects on organic carbon stabilization in aggregates of Mollisols. J. Food Agric. Env. 11, 1164–1168 (2013).
Chung, H., Ngo, K. J., Plante, A. & Six, J. Evidence for Carbon Saturation in a Highly Structured and Organic-Matter-Rich Soil. Soil Sci. Soc. Am. J. 74, 130–138 (2010).
Feng, W. et al. Testing for soil carbon saturation behavior in agricultural soils receiving long-term manure amendments. Can. J. Soil Sci. 94, 281–294 (2013).
Acknowledgements
This study was granted by Special Fund for Agro-scientific Research in the Public Interest of China (No. 201503116), National Key Research and Development Program of China (2016YFD0300809), National Natural Science Foundation of China (41725004), and by Innovation Program of the Institute of Soil Science CAS (No. ISSASIP1610).
Author information
Authors and Affiliations
Contributions
X.H.P. conceived of this study. Z.C.G., Z.B.Z. and H.Z. collected the field soil samples. D.Z.W. conducted the crop yield analysis and managed the experiment. Z.C.G. performed the experiments, analysed the data, and wrote the manuscript. Z.C.G. and X.H.P. contributed substantially to the manuscript revisions.
Corresponding author
Ethics declarations
Competing Interests
The authors declare no competing interests.
Additional information
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Guo, Z., Zhang, Z., Zhou, H. et al. The effect of 34-year continuous fertilization on the SOC physical fractions and its chemical composition in a Vertisol. Sci Rep 9, 2505 (2019). https://doi.org/10.1038/s41598-019-38952-6
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/s41598-019-38952-6
This article is cited by
-
Stabilization mechanisms of organic matter in Andosols under long-term fertilization as revealed from structural, molecular, and stable isotopic signatures
Journal of Soils and Sediments (2024)
-
Contrasting effects of straw and straw-derived biochar application on soil organic matter and corn yield in a Chinese Mollisol
Journal of Soils and Sediments (2023)
-
A Strategy for Reducing Nitrogen Fertilizer Application Based on Application of Biochar: A Case in Northeast China Black Soil Region (Mollisols)
Journal of Soil Science and Plant Nutrition (2023)
-
Influence of small-scale spatial variability of soil properties on yield formation of winter wheat
Plant and Soil (2023)
-
How do terrestrial plants access high molecular mass organic nitrogen, and why does it matter for soil organic matter stabilization?
Plant and Soil (2021)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.