Carbon and nitrogen pools in different aggregates of a Chinese Mollisol as influenced by long-term fertilization

Purpose It is known that soil organic matter (SOM) dynamics are sensitive to fertilizations, but it is different from soil to soil. It is unclear how the long-term applications of organic manure and mineral fertilizers impact the accumulation and distribution of soil organic carbon (SOC) and total nitrogen (TN) especially in soil aggregate fractions of Chinese Mollisols, which have been intensively cultivated for decades under maize monocropping and conventional tillage ways. Thereby, the research of this kind is very important for the sustainable use of agricultural land in China, where land resources are extremely limited for its huge population. The objectives of this study were to identify how the long-term fertilization treatments would affect the aggregate, SOC and TN distribution pattern in the Chinese Mollisol, and how soil aggregation contribute to the storage and stabilization process of SOC and TN. Materials and methods Soils were sampled in April 2005 from a long-term experiment site established in 1979. Six treatments were selected as follows: CK (zero fertilizer), NPK (mineral fertilizers), M1 (organic manure at lower level), M1NPK (combination of M1 and NPK), M2 (organic manure at higher level) and M2NPK (combination of M2 and NPK). Four aggregate fractions, i.e., large macroaggregate (> 2,000 A mu m), small macroaggregate (250-2,000 A mu m), microaggregate (53-250 A mu m) and silt + clay fraction (< 53 A mu m), respectively, were obtained by wet sieving. SOC and TN in bulk soils and aggregates were analyzed using a C/N/H/S-analyzer (Vario ElementarIII, Germany). Results and discussion The small macroaggregate was the most abundant component in the soils, accounting for 46.1-60.2% of the dry soil mass and about 50% of SOC and TN. The small macroaggregate proportion decreased while large macroaggregate percentage increased significantly after organic manure treatment. However, mineral fertilizers application enlarged the proportion of microaggregate and silt + clay fraction. Consequently, SOC and TN contents in all the four aggregates increased remarkably with organic manure application, whereas mere mineral fertilizers addition did not affect SOC and TN content significantly. The combination of NPK with the higher level of organic manure was more effective in improving SOC and TN accumulation than organic manure application alone. Moreover, organic manure application tended to decrease C/N ratios, in general, and higher C/N ratios in > 53 A mu m fractions than in others, suggesting that SOC in the larger fractions is relatively fresh or little altered by microorganisms, whereas, the SOC associated with < 53 A mu m fraction is more decomposed and relative aged. However, the higher enrichment factor for N (E (N)) than for C (E (C)) in < 53 A mu m fraction indicates a preferential depletion of labile SOC and relative accumulation of stable SOC pools in the finest fraction. Conclusions The long-term application of organic manure generally facilitated aggregation processes and then improved the quantity and quality of SOM in the Mollisol, whereas the long-term mineral fertilizers application generally resulted in the de-aggregation effect. The combination of organic manure and mineral fertilizers was a better way to further enhance the sequestration of SOC and improve SOM quality by aggregation compared to the pure organic manure. It is concluded that when increasing organic input, physical protection plays a major role for the sequestration of the increased SOC in the Chinese Mollisol. However, when SOC is depleted under a certain management practice, it will happen preferentially in the physical protected SOC in large aggregates. This study has great implications for better understanding SOC and N dynamics as affected by long-term fertilization.