Effect of organic matter input on functional pools of soil organic carbon in a long‐term double rice crop experiment in China

Abstract

It is known that crop residue input has a strong impact on the quantity of soil organic matter (SOM). However, its influence on SOM quality is often unknown. The effect of organic matter (OM) input on soil organic carbon (SOC) and functional SOC pools was investigated in a Chinese long‐term double‐cropping experiment with rice. It could be demonstrated that the C content of hot‐water soil extracts (CHWE) may serve as an indicator of the decomposable SOC pool. A particle size–density fractionation was used to separate SOC pools with different interactions with the mineral soil matrix. The hot‐water C extractability of different particle size‐density fractions, together with their δ13C isotopic signatures, was applied to differentiate stability and potential degradability of SOC pools. The main findings were: first, the input of OM (rice straw + animal manure + green manure) to the unfertilized (Nil) and fertilized (NPK) treatments resulted in a 43–64% increase of SOC over 14 years. Second, during the same period the CHWE pool declined by 25–32% in plots without OM input. Organic amendments reduced the decrease in the CHWE pool by about 50%. Hot‐water‐extractable carbon (C) (CHWE) of bulk soil could serve as an indicator of decomposable SOC pools with the potential for nutrient supply during decomposition. Third, the importance and capacity of smaller‐sized clay particles (CF1 <1 µm) for C storage was proved by large C enrichment factors (ESOC). Fourthly, functional C pools in different size‐density fractions could be clearly separated from each other according to their stability, by relating the 13C isotopic abundance of C pools to the hot‐water‐extractable component (δ13C*fraction).