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在美国七个地方的长期耕作对比试验中,土壤微生物生物量和潜在性可矿化氮(PMN)的分布随耕作制和土层深度而发生变化。在免耕土壤中,微生物生物量和PMN水平相对于犁耕土壤表层,平均高出54%和37%。在免耕土表0—7.5cm内,生物量和PMN水平最高,并随士层深度增加而降低直至30cm。而在犁耕土壤中,生物量和PMN水平一般在7.5—15cm范围内最高。由于耕作制的影响,微生物生物量与土壤全C和全N、含水量及水溶性碳的分布有密切的关系。而潜在性可矿化氮含量主要与微生物生物量和土壤全N的分布有关。PMN和微生物生物量的绝对水平及其因耕作制的不同而产生的相对性差异取决于各实验点的气候、种植方式与土壤条件。在7处实验点中的6个点上,和犁耕土壤相比,在免耕土壤表层0—7.5cm内,土壤生物量和PMN中所增加的N分别为13—45和12—122kgN/ha。肥料施在富含生物性物质的土壤表层,或轮作可提高速效N的利用和少耕土壤中作物的生长。
In long-term farming comparisons in seven locations in the United States, the distribution of soil microbial biomass and potential mineralizable nitrogen (PMN) varied with tillage and depth of soil. In no-till soil, microbial biomass and PMN levels were on average 54% and 37% higher than those of plowed soil. Within 0-7.5cm of no-till soil surface, the biomass and PMN level were the highest, and decreased to 30cm with the depth of stratum increasing. In plowed soil, biomass and PMN levels are generally highest in the 7.5-15 cm range. Due to the tillage system, microbial biomass is closely related to the distribution of soil total C, total N, water content and water-soluble carbon. The potential mineralizable nitrogen content is mainly related to microbial biomass and soil N distribution. The absolute levels of PMN and microbial biomass and their relative differences due to tillage systems depend on the climates, planting patterns and soil conditions at each experimental point. At 6 of the 7 experimental sites, compared with the plowed soil, the added N in soil biomass and PMN in the 0-7.5 cm of no-tillage soil surface were 13-45 and 12-122 kg N / ha Fertilizers applied to the surface of soils rich in biological matter, or crop rotation, may enhance the use of available N and the growth of crops in less-tilled soils.