Effect of long-term addition of organic substances on soil nitrogen and amino sugars in particle-size fractions in the North China Plain

The North China Plain (NCP) is one of the main grain production bases in China. In pursuance of high yield, excessive application of chemical fertilizers has been becoming increasingly common and problematic, especially in terms of soil quality degradation. A long-term experiment on soil organic nutrient recycling was conducted at Luancheng Agroecosystem Experimental Station, Chinese Academy of Sciences in this study. The aim was to evaluate the effects of different exogenous organic materials application on the distribution of nitrogen and amino sugar in different soil particle-size fractions and to provide the basis for explaining the soil physical and biological protection mechanisms of nitrogen under different management practices. The experiment had six treatments in triplicates-no chemical fertilizer without organic materials (CK), single application of pig manure (M), application of chemical fertilizers (NPK), combined application of chemical fertilizers and pig manure (MNPK), combined application of chemical fertilizers and straw (SNPK) and single application of straws (SCK). Three particle-size fractions (sand:2 000-53 μm, silt:53-2 μm and clay: < 2 μm) were separated through ultrasonic dispersion and centrifugal separation. Total nitrogen and amino sugars (glucosamine, muramic acid and amino galactose) were analyzed within these fractions and in the bulk soil. Based on the stability and heterogeneity of three soil amino sugars, we used amino sugar as index for fungal/bacterial residue accumulation and transformation to explain the role of fungi/bacteria in nutrient conversion. The results indicated that the addition of organic materials (straw and pig manure) significantly increased contents of total nitrogen and amino sugars in bulk soil and different particle-size fractions with order of clay fraction > sand fraction > silt fraction. The effect of adding exogenous organic substances on soil nitrogen content was most obvious in sand fraction. Soil nitrogen in MNPK treatment was enriched mainly in sand fraction, and nitrogen in SNPK was most enriched in clay fraction. Straw addition increased soil content of glucosamine from fungal residue, pig manure application increased content of muramic acid from bacteria residue, indicating obvious effect of exogenous organic substances on community structure of soil microorganisms. The ratios of glucosamine/muramic of soil particle-size fractions showed that bacteria dominated under pig manure application, and fungi dominated under CK or straw addition in nutrient decomposition and transformation, which was most obvious in sand fraction. In summary, partial replacement of chemical fertilizers with organic manure not only reduced chemical fertilizer use, but also increased soil nutrient content, improved microbial community structure and soil quality.