Soil Organic Carbon Turnover Under Straw Return

Soil organic carbon (SOC) plays a critical role in global C cycling and soil functions. Maintaining and increasing SOC has been considered a win-win strategy for mitigation of greenhouse gas emissions and sustainability of crop production. As one of the few management options to alter the agroecosystem C inputs, straw return has been widely recommanded to maintain soil fertility and crop productivity in China, but effects of long-term straw incorporation on crop yield, stocks of soil organic carbon and total nitrogen (TN), components of SOC and its turnover rate have not been quantitively evaluated.This study tried (1) to quantify the effect of long-term continuous straw return on crop yield, SOC and TN stocks across the major agricultural zones of China; (2) to summary the decomposition rate and fraction of C mass remaining of straw as well as crop roots and manure in short-term (1-8 years) field incubation; (3) to analyze the effect of straw and fertilizer on the components and turnover rate of SOC; and (4) to evaluate the DayCent model based on long-term straw return trials and explore the site-specific factors. The main results and conclusions were as follows:1. Based on data collected from long-term (> 10-year) trials across the major agricultural zones of China, we found that straw return significantly increased crop yield, SOC and TN (by 7.0,10.1 and 11.0%, respectively) as compared to straw removal. In some trials with winter wheat in northern China, straw return reduced yield by 0.6-7.1%. The effects of straw return on SOC and TN were not significantly affected by experimental duration, land use type and cropping system, but positively and linearly related to the inputs of straw-C and -N, respectively. Interestingly, SOC and TN responses to straw return were decoupled in upland and upland-paddy soils in China, but not in paddy soils. Mean values of straw-C sequestration efficiency (7.7,10.3 and 9.4%, under corn, wheat and rice, respectively) indicate that 100% straw return could increase SOC by 281.7 Tg C in 18 years (the mean experimental period of the considered studies) in China. Our analyses demonstrate that straw return is an effective practice for sustaining crop productivity and soil fertility in large parts of China, but site-specific factors should be considered.2. Data was collected from the 56 published studies during 1980—2013 that have examined the decomposition of organic materials across China. Overall, fraction of C remaining after one year’s decomposition (hi) ranged from 0.10 to 0.75 g g-1, with an average of 0.335 ± 0.005 g g-1 across China, and was significantly affected by types of organic materials and agricultural regions and their interactions. On the whole, in terms of hi, the four major types of organic materials displayed an order of green manure< straw< root and organic manure, and the regions, an order of South China ≈ North China< Northwest China ≈ Northeast China. However, the regional variation of hi was also affected by type of organic materials. For instance, hi of straw, root and green manure were lower in North China than in South and Northeast China, while hi of organic manure did not differ much between regions. On the other hand, h1 of organic materials also varied with the regions. For example, in humid and semi-humid Northeast, North and South China, h1 of root was higher than that of green manure and straw, while in the arid and semi-arid Northwest China, hi did not differ much between types of organic materials, because of aridity. For a longer period, the fraction of mass remaining would decrease to a stable, which was lower for straw (0.16-0.20) and manure (0.21) than for roots (0.30-0.33). The decomposition process was fit well by two-pool models when cumulative temperature as an indicater. Litter quality can only donimate the decomposition process in a short period (0.6-1.5 years), but can not be used to predict the long-term retention. Nutrient addition also cannot significantly increase the long-term C retention.3. Based on a meta-analysis of stable 13C in long-term (6-49 years) trials across the temprate region, we found that new C (corn-SOC) account for 22.3% of total SOC with a turnover rate of 0.105 yr-1, while native SOC account for 77.7% with a much slower turnover rate of 0.0089 yr-1. Long-term continuous straw return and/or fertilizer significantly increased stocks (31-130%), proportion (2.8-19.2 unit) and turnover rate (125%) of new C. Straw return also accelerated the turnover rate of native SOC (by 74%). Positive priming effect of straw return largely explained why the response of SOC to straw return was not as high as expected. If we consideded the priming effect, the gross C sequestration efficiency of corn straw could be twice of net C sequestration efficiency, i.e., to reach 15%, which was similar to the stable level of com straw remaining after 1-3 years of field incubation, suggesting that crop straw can be converted to stable SOC with a very short-term.4. DcyCent model did a good job to simulate the dynamics of SOC and TN in treatments with C input by roots and manure, but could over-estimate SOC stocks when great amount of straw returned. The inverse modelling showed that the magnitude of site-specific coefficiency, a key factor to modify the decomposition rate constant of SOC, was significantly corrected to the amount of straw-C input, and also affected by other site factors, e.g., the way of straw return (incorporated or surface covered), nutrient balance of soil-plant system and saturation defiency of mineral-associated SOC.