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The contents of soil N fractions under different mulching. Values are means ± standard error (n = 3). Different letters indicate significant differences (p < 0.05) between the treatments. SM, straw mulching; FM, plastic film mulching; and CK, no mulching. MBN, microbial biomass N; PNM, potential N mineralization; PON, particulate organic N; STN, soil total N; NH4+-N, ammonium; NO3−-N, nitrate N.
