Farmers in semi-arid regions are experimenting with cover crop mixtures as partial summer fallow replacement, with the goal of increasing soil biological activity. We compared extracellular enzyme activity, microbial biomass, potentially mineralizable nitrogen (PMN) and mycorrhizal colonization levels after growing cover crops comprised of one or four functional groups. We asked whether cover crops can alter those soil processes and components relative to fallow during the cash crop growing season, and whether a cover crop mixture (CCM) will have a greater impact than a single-species legume green manure (LGM) after one and two rotations with wheat. Cover crops were planted at two sites in Montana April 2012 and May 2014, and at two additional sites in May 2013, and April 2015, terminated at each site approximately 60 days after seeding. Wheat was the cash crop for the following growing season. Cover crop treatments included a fallow control, a sole pea LGM, and a FULL mix consisting of eight species, with two species of each of four functional groups. Four additional treatments of single functional groups were compared following the second rotation of cover crops. The geometric mean of soil enzyme activity increased after cover crops relative to fallow (1.1–1.3 fold) at two sites after the first rotation, and by 1.1–1.4 fold after the second rotation at three sites. At the fourth site, the geometric mean of the FULL mix was greater than LGM or fallow by 1.4 and 1.5 fold. The response of individual enzymes was less consistent. Microbial biomass and PMN increased after LGM and/or FULL at three of eight site-years, by 1.2–1.7 for microbial biomass and 1.6–2.1 for PMN. Mycorrhizal colonization of wheat increased in at least one cover crop treatment relative to fallow at four of six site-years, and decreased in cover crops relative to fallow at one site. There were no differences in soil biological traits measured among single-functional-group treatments. Soil biological parameters were correlated with cover crop biomass in 7 of 16 comparisons at two sites after one rotation. After two rotations, 5 of 8 soil parameters were correlated with cover crop biomass at one site, with no correlations for the remaining sites. Precipitation declined 30–50 % from the first rotations at those three sites. Positive effects of cover crops on soil biological parameters in semi-arid systems may be measurable only when environmental conditions do not limit growth.

半干旱地区的农民正在尝试使用遮盖作物混合物作为夏季休耕的部分替代品，目的是提高土壤生物活性。在生长由一个或四个功能组组成的覆盖作物后，我们比较了细胞外酶活性，微生物生物量，潜在可矿化的氮（PMN）和菌根定植水平。我们询问在经济作物生长季节，覆盖作物是否可以改变土壤过程和土壤成分，与休耕相关，以及覆盖作物混合物（CCM）是否会比单种豆科植物绿肥（LGM）产生更大的影响？小麦旋转两次。覆盖作物分别于2012年4月和2014年5月在蒙大拿州的两个地点播种，并于2013年5月和2015年4月在另外两个地点播种，播种后约60天在每个地点终止。小麦是接下来生长季节的经济作物。覆盖作物的处理措施包括休耕控制，豌豆LGM和由8种组成的FULL混合饲料，其中四个功能组各有2种。在第二次轮作覆盖作物之后，比较了单个功能组的四种其他处理方法。覆盖作物后，土壤酶活性的几何平均值相对于第一轮旋转后两个位置的休耕期增加（1.1-1.3倍），而第二轮旋转后三个位置则增加1.1-1.4倍。在第四个位置，完全混合的几何平均值比LGM或休假大1.4和1.5倍。个别酶的反应不太一致。LGM和/或FULL在八个站点年中的三年后，微生物生物量和PMN增加，微生物生物量增加1.2-1.7，PMN增加1.6-2.1。相对于休耕，小麦的菌根定植在至少一种覆盖地处理中在六个位点年中的四个年均增加，而在覆盖作物中相对于休耕地在一个站点处有所减少。在单功能组处理之间测得的土壤生物学特性没有差异。一轮旋转后，在两个地点的16个比较中，有7个土壤生物学参数与覆盖作物生物量相关。两次旋转后，在一个站点上将8个土壤参数中的5个与覆盖作物生物量相关联，而在其余站点上则没有相关性。与这三个地点的第一次旋转相比，降水量下降了30–50％。只有在环境条件不限制生长的情况下，才可以测量覆盖作物对半干旱系统中土壤生物学参数的积极影响。