Waterlogging accelerates the loss of soil organic carbon from abandoned paddy fields in the hilly terrain in subtropical China

Paddy soils have been widely recognized as important carbon sinks. However, paddy field abandonment is increasing in the hilly area in subtropical China. Soil waterlogging and weed burning are common practices in abandoned paddy fields, which could affect vegetation cover and carbon sequestration. An rice cultivation experiment was ceased in 2006, and four new treatments were applied as waterlogging (W), drainage (D), waterlogging combined with burning (WB), and drainage combined with burning (DB). Waterlogging altered the vegetation cover and caused an associated change in biomass. Paspalum paspaloides, Murdannia triquetra, and Bidens frondosa dominated W and WB plots, and Microstegium vimineum and Bidens frondosa dominated D and DB plots. Abandonment of paddy fields led to a rapid decrease in soil organic carbon (SOC), and waterlogging accelerates SOC loss which should be attributed mainly to alteration of the vegetation cover. Six years’ rice cultivation increased SOC content by 13.5% (2.4 g kg−1) on average. In contrast, six years’ abandonment reduced SOC content by 14.5% (3.0 g kg−1) on average. Decline rate of SOC was 0.38, 0.64, 0.30, and 0.65 g kg−1 a−1 for D, W, DB, and WB, respectively. Such results indicate a significant risk of SOC loss from abandoned paddy fields.

rice production in recent decades and retarded decomposition under anaerobic conditions 11 . Increasing number of studies have reported that abandoned agricultural land will be occupied by natural vegetation and lead to SOC accumulation 5,8,12,13 . However, these studies focused on abandoned dryland. To our knowledge, change of SOC content in abandoned paddy fields in the subtropical area has been rarely reported. Shift from paddy fields to uncultivated land will result in dramatic change in soil environment. Field management, such as tillage, fertilization, irrigation, etc. will be ceased, and vegetation will be changed from crops to native vegetation. All these changes will lead to alteration in organic carbon input and decomposition, as well as other nutrient transformations 14,15 .
Soil waterlogging and weed burning are common practices in abandoned paddy fields. In hilly regions, rice paddies are generally located in low-lying areas, and some abandoned paddy fields become waterlogged area due to poor drainage. Abandoned paddy fields are occupied by natural vegetation. And weed burning in winter is a common practice in abandoned paddy fields to avoid woody plant colonization, in case of reutilization of such land for crop land in the near future. Soil waterlogging and weed burning may alter vegetation cover in abandoned paddy fields and cause an associated change in belowground roots, which will affect organic carbon input. At the same time, soil water content will exert an impact on decomposition and mineralization of soil organic matter, thus further influencing the accumulation of soil organic carbon 11,16,17 . Additionally, the weed burning of aboveground weed residues may also influence the accumulation of soil organic carbon.
In this study, we established an abandoned rice paddy experiment with four treatments, including waterlogging, drainage, waterlogging combined with burning, and drainage combined with burning. The objective was to examine the effects of soil waterlogging and weed burning on vegetation and SOC content in abandoned paddy fields. The root biomass in the soil layer of 0~20 cm varied among different treatments from 219 g m −2 to 549 g m −2 (Fig. 2). The root biomass was mainly distributed in the surface soil layer of 0~5 cm, accounting for 66~80% of that in the soil layer of 0~20 cm. Waterlogging increased belowground biomass.  SCIentIfIC RePoRTS | 7: 14549 | DOI:10.1038/s41598-017-14820-z Soil organic carbon. In the six years before paddy field abandonment, rice cultivation increased SOC by 13.5% (2.4 g kg −1 ) on average in the twenty plots (Fig. 3). In contrast, SOC contents in every plots decreased after the field abandonment for six years. From 2006 to 2012, the SOC content decreased by 14.5% (3.0 g kg −1 ) on average in the twenty plots, which indicated that organic carbon input into the soil was far lower than SOC mineralization in the abandoned rice paddies. Weed burning had little effect on SOC content. In contrast with drainage, waterlogging accelerated SOC loss. SOC contents under W and WB were significantly lower than those under D and DB (P < 0.05). Decline rate of SOC was 0.38, 0.64, 0.30, and 0.65 g kg −1 a −1 for D, W, DB, and WB, respectively.

Discussion
Paddy soils have been widely recognized as important carbon sinks 1,2,18,19 . In the present study, SOC content increased by 0.4 g kg −1 a −1 in the field of double rice cultivation for six years. In striking contrast, SOC content decreased by 0.5 g kg −1 a −1 following an abandonment for six years. During the stage of rice cultivation, around 4,000~5,000 kg ha −1 rice residues (root and stubble) and 2,000~3,000 kg ha −1 weed biomass (weed growing in fallow season) were incorporated into soil every year according to field observation. Also, a 14 C labeling study showed that a considerable amount of rice root exudation of photosynthesis-derived carbon (account for 8.0~19.3% of rice biomass carbon) was incorporated into SOC pool 20 . During the stage of abandonment, weed root biomass ranged from 2,500~6,000 kg ha −1 (Fig. 2). Obviously, organic carbon input in abandoned paddy soil was much lower than that in cultivated paddy soil. On the other hand, flooded conditions restricted decomposition of fresh  organic materials 11,16,17 . The soil flooding period was relatively shorter in the stage of abandonment than that in the stage of rice cultivation, leading to the decomposition of more organic materials in abandoned paddy fields.
Similarly, some researchers reported that land use change from paddy rice cultivation to grassland, woodland, and upland crop cultivation caused SOC loss [21][22][23] . It has been widely recognized that land use changes from forest or grassland to crop fields lead to SOC loss, and the reverse processes lead to SOC accumulation 5,8,12,13 . However, rice paddy ecosystems are quite distinctive in SOC accumulation compared with other ecosystems in subtropical China 2 . In contrast, a study conducted in tropical Brazil showed that paddy fields loose much SOC when compared to native vegetation in the tropics 24 . That was because SOC content of the tropical forest soil (30~63 g kg −1 ) was way higher than that of the paddy field soil, even higher than SOC saturation content of the paddy field soil (26~28 g kg −1 ) 4,24 . The results suggest that whether SOC storage shows a positive or negative change depends on the capacity of an ecosystem to accumulate SOC when land use change occurs.
In the present study, waterlogging accelerated SOC loss, whereas weed burning had little effect on SOC loss. Possible reasons are ascribed as follows. First, waterlogging greatly changed vegetation composition ( Table 1)    Besides, much root from perennial plants under waterlogging conditions may slow down the regeneration of root, and consequently reduce the input of dead roots into soil. Second, weed burning increased aboveground biomass, but tended to decrease belowground biomass, the integrated effects of which played down the impact of burning on SOC. Correlation analysis showed that SOC decline rate was positively correlated with the initial SOC content (SOC content in 2006) (r = 0.697, P < 0.01, Fig. 4a), indicating higher SOC decline rate for soils with higher SOC content. SOC decline rate was positively correlated with the TN decline rate (r = 0.590, P < 0.01, Fig. 4b), indicating a risk of nitrogen loss with decreasing of SOC. In subtropical China, rice paddy ecosystems have high SOC content, and abandonment of rice paddy fields tends to cause SOC loss accompanied by soil nitrogen loss. At present, China is still facing the challenge to feed the growing population. It is recommended that abandoned rice paddy fields be reutilized to avoid soil carbon and nitrogen loss and to ensure food safety.

Materials and Methods
Field trial. The investigation was conducted at Taoyuan Station of Agro-ecology Research in Taoyuan County, Hunan Province (28°55′N, 111°27′E). The region is characterized by a subtropical humid monsoon climate, with an annual average of air temperature, precipitation, sunshine, and frost-free period of 16.5 °C, 1,448 mm, 1,513 h, and 283 days, respectively. The region is featured with low hilly areas, with relative height less than 100 m. Gentle slope lands (6°~15°) account for 63% of total area. Many small and fragmented farmlands, accounting for 14%, are distributed on valley floor, the slope of which is generally smaller than 6°. The soil is developed from Quaternary red clay.
A double rice (two rice crops per year) field experiment was established in randomized block design with five fertilization treatments and four replicates for each treatment in 1994. The soil has been subjected to paddy farming for more than 300 years. It was flooded rice cultivation with tillage for every cropping season. The fertilization treatments were NPK, NK, NP, PK, and control (N, Nitrogen; P, phosphorus; K, potassium). Each plot was 7.5 m × 4.5 m in size. All the plots were arranged at a plane of the same altitude and surrounded by cement walls. The plots were not used for rice cultivation any more after the late-rice harvest in 2006. The twenty plots were divided into four groups, and each group comprised five plots which used to be one replicate of NPK, NK, NP, PK, and control, respectively. The four groups were arranged with four treatments which were waterlogging (W), drainage (D), waterlogging combined with weed burning (WB), and drainage combined with weed burning (DB). The five plots in each treatment was treated as replicates. Waterlogging meant collecting rainfall without drainage. The W and WB plots were usually saturated or flooded from April to August during which there was plenty of precipitation. In contrast, there was a water outlet in the cement wall for drainage, so the D and DB plots were non-flooded. Weed burning was implemented in December every year. The plant aboveground was burned to ash. Monthly averages of precipitation and air temperature from 2007 to 2012 were presented in Fig. 5.

Vegetation investigation.
Vegetation investigation was carried out in July 2012 and July 2013. Three quadrats (1 m × 1 m) were set in each plot to investigate the height, density, coverage, and aboveground biomass for each plant. Dominant species were determined on the basis of Importance Value (IV), which was calculated as IV = (relative height + relative frequency + relative coverage)/3 25 . Three columns (20 cm length × 20 cm width × 20 cm depth) were collected in each plot to investigate the belowground biomass. Each column was split into 0~5 and 5~20 cm depths to measure the vertical distribution of weed root biomass. The samples were dried at 80 °C, and weighed to calculate their biomass.
Measurements. Soil samples from the surface layer (0~20 cm) were collected in December in 2000, 2006, and 2012. Six soil cores were collected from each plot using augers and mixed to provide a single sample. The visible pieces of plant residues (>2 mm) were removed. Soil organic carbon (SOC) was determined by dichromate oxidation method 26 . Dichromate oxidation method has been widely used in soil investigations because of its simplicity and rapidity, although it is subject to interference by oxidizable or reducible soil constituents such as Cl − , Fe 2+ , and MnO 2 26 . Soil total nitrogen (TN) was determined by the Kjeldahl method 27 . Soil samples (0~20 cm, in 2006) contained 20.1 g kg −1 SOC, 1.78 g kg −1 TN, 0.76 g kg −1 total P, 15.2 g kg −1 total K, 14.9 mg kg −1 Olsen P, and 58.3 mg kg −1 available K in average. SOC decline rate (g kg −1 a