Specific Responses of Soil Microbial Residue Carbon to Long-Term Mineral Fertilizer Applications to Reddish Paddy Soils

Mineral nutrient inputs to soil may alter microbial activity and consequently influence the accumulation of microbial residues. In this study, we investigated the effects of application rates and ratios of mineral fertilizers on the microbial residue carbon (MRC) of reddish paddy soils after long-term (15-year) fertilizer applications in southern China. Contents of three soil amino sugars as microbial residue contents were determined and MRC were calculated based on amino sugars. Results showed that three individual amino sugar contents increased as fertilizer application rates increased until maximum values were reached at a rate of 450-59-187 kg ha−1 year−1 (N-P-K). The three amino sugar contents then declined significantly under the highest mineral fertilizer application rate of 675-88-280 kg ha−1 year−1 (N-P-K). In addition, to enhance the microbial residue contents, it was more beneficial to double P (N:P:K = 1:0.26:0.41) in fertilizers applied to the P-deficient reddish paddy soils than to double either N (N:P:K = 2:0.13:0.41) or K (N:P:K = 1:0.13:0.82). The contents of the three individual amino sugars and microbial residues under different fertilizer application rates and ratios were significantly and positively correlated with soil organic carbon (SOC), total N, total P, and pH. Increases in values of the fungal C to bacterial C ratios showed that soil organic matter (SOM) stability increased because of the fertilizer applications over the past 15 years. The contents and ratios of amino sugars can be used as indicators to evaluate the impact of mineral fertilizer applications on SOM dynamics in subtropical paddy soils. The results indicated that fertilizer applications at a rate of 450-59-187 kg ha−1 year−1 (N-P-K) may improve crop yields, SOC contents, and SOC stability in subtropical paddy soils.