Turnover And Transformation Mechanisms Of Organic Carbon Poolin A Black Soil Under Long-Term Fertilization

The black soil of Northeast China, which is known as one of the three largest black soil zones in the world and the major grain producing areas in China, has a great agricultural production capacity and potential. In recent years, the degeneration of black soil is becoming more and more serious, such as decline in soil fertility, decrease in soil organic matter (SOM) and organic carbon (SOC) year by year. However, the SOC would be improved effectively and maintained to a certain level through implementation of rational fertilization. Previous studies conducted aimed at the apparent changes in soil organic carbon pool, however, current study will use the natural abundance of stable C isotope combined with the SOM fractionation technique to clarify the turnover and transformation mechanisms of organic carbon pool fer black soil under long-term fertilization, through two aspects of the SOC input and output The study will not only help quantify the dynamic of SOC storage and confirm the optimal fertilization measures of black soil in Northeast China, but has a profound meaning on the improvement of land productivity and food safety in China. The main conclusions are as following:(1) The effects of long-term fertilization on soil organic carbon dynamics in a black soil of Northeast China. We combined soil chemical fractionation with stable C isotope analyses to investigate the C dynamics of the various SOC pools after 25 years of fertilization. We found that 25 years of fertilization significantly changed the dynamics of soil organic pool, recalcitrant pool and labile pool, and long-term fertilization had a positive overall impact on recalcitrant C (RC) accumulation, regardless of the application of inorganic or organic fertilizers. Chemical fractionation showed long-term MNPK fertilization strongly increased the SOC storage in both soil layers (0-20 cm= 1492.4 g C m2 and 20-40 cm= 1770.6 g C m2) because of enhanced RC and labile C (LC). Moreover, the positive effect of manure addition through the return of belowground biomass or the direct amendments of organic manure with a high SOC content was not offset by the soil C decomposition in MNPK-treated soils. Thus, MNPK fertilizer was shown to be the most effective measure for soil C sequestration in the longer term. The 25 years of inorganic fertilizer treatment did not increase the SOC storage mainly because of the offsetting effects of enhanced RC and decreased LC, whereas no clear SOC increases under the SNPK fertilization resulted from the fast decay rates of soil C (e.g. labile C). Interestingly, corn straw combined with inorganic fertilizers could accelerate the soil C turnover when compared with the simple addition of inorganic fertilizers or straw alone.(2) The effects of long-term fertilization on dynamics of soil organic carbon fractions in a black soil of Northeast China, we build on our previous findings and utilize the natural abundance of 813C together with soil physical fractionation technique to evaluate dynamics in the SOC fractions after 25 years of fertilization.25 years of fertilization significantly increased the SOC storage, mainly by enhancing the soil C of the macroaggregates (2000-250 ?M) with most of the C and N stored in the intra-aggregate particulate organic matter (iPOM) (76.87%) in the black soils. Overall, application of N and NPK fertilizers cannot significantly increase the SOC storage but enhanced C in mineral-associated organic matter (mSOM) of aggregates, whereas MNPK fertilizer resulted in the greatest amount of SOC storage (about 5221.5 gC m2) because of the enhanced SOC in light fraction (LF), iPOM and mSOM of each aggregate. Additionally, the improvement of SOM in MNPK-treated soils maybe first ascribed to a decline of C/N ratios in LF. The SNPK fertilizer increased SOC storage in> 250 ?m aggregates but reduced SOC storage in< 250 ?m aggregates due to SOC changes in LF and iPOM. Physically fractionation further confirmed that SNPK fertilizer could accelerate the soil C turnover, largely through various sizes of soil aggregates including macroaggregates and microaggregates. The present results further confirmed the previous study conducted by soil chemical fractionation technique and suggested that long-term application of fertilization could alter the SOC storage, consequently affecting the dynamics of soil C pools in agro-ecosystem.(3) Long-term trials of effects of different cropping systems on dynamics of soil organic carbon fractions in a black soil of Northeast China. Our results indicated that 25 years of continuous maize cropping (MM) and maize-soybean rotation (MS) with manure application significantly increased the SOC storage, mainly by enhancing the soil C of the macroaggregates (2000-250 ?M) with most of the C stored in the iPOM (62.01-90.32%) in the black soils of northeast China. We found that enhanced organic C was largely controlled by the amount of each aggregate size and was ascribed to the increased SOC in all density fractions (LF, iPOM, and mSOM) of each aggregate under both of cropping systems. Additionally, long-term maize-soybean rotation was more favorable for the SOC accumulation of macroaggregates (> 250?m), while continuous maize cropping may be a more effective measure for soil C sequestration in a long-term period because of more stable C stored in the small size fraction (< 53um). The faster C turnover rate of soil fractions occurred in MS system than in MM system, maybe related to a high C:N ratio of maize residue as well as nitrogen-fixation of soybean. More interestingly, we found maize (C4)-derived SOC can largely affect the whole soil C turnover level and be proportional to soil C turnover rate under 25 years of MS (C4-C3) rotation system. Our study highlighted the importance of soil C dynamics under long-term cropping systems, which would help to estimate the future C sequestration in agricultural lands.