The influence of long-term animal manure and crop residue application on abiotic and biotic N immobilization in an acidified agricultural soil

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Long-term application of organic fertilizers in acidified agricultural soils could alter soil nitrogen (N) immobilization by providing carbon (C) source and alleviating soil acidification. However, an understanding of the relative importance of abiotic and biotic N immobilization in acidified agricultural soils following long-term organic fertilizer application is largely lacking. Generally, the application of crop straw, which has a higher C/N ratio than animal manure, results in higher biotic N immobilization in soils. In contrast, the application of animal manure can result in greater stimulation of soil microbial activity due to a higher capacity to alleviate acidification. Resolving this contradiction is critical for predicting microbial N retention capacity and the effects of different types of organic fertilizer in acidified agricultural soils. A laboratory 15N tracer experiment was carried out to examine the effects of long-term animal manure and crop straw application on abiotic and biotic N immobilization in an acidified agricultural soil in China. Animal manure application had a higher stimulation effect on abiotic and biotic N immobilization than crop straw. Microbial NH4+-N immobilization, as opposed to abiotic NH4+-N immobilization, largely contributed to the retention of NH4+-N. In contrast, abiotic NO3−-N immobilization had a more significant role in NO3−-N retention than microbial NO3−-N immobilization under elevated C conditions. Animal manure-induced increases in microbial NH4+-N and NO3−-N immobilization could be attributed to both increased C availability and enhanced soil microbial activity due to an increase in soil pH. The results suggest that long-term inputs of organic material to acidified agricultural soils could enhance abiotic and biotic N immobilization capacity.