*3.1. Cover Crop Biomass and Nitrogen Content*

Although nitrogen concentrations in aerial parts of the cover crops were not significantly different, due to higher biomass production sunn hemp (SH) and sunn hempcrimson clover mixture (SH + CC) produced significantly more biomass than either monocrop crimson clover (CC) or weedy fallow (NC) (Table 2, Figure 1). Therefore, their aboveground biomass residues added more nitrogen to the soil when incorporated before barley planting (Table 2, Figure 2). SH produced the highest amount of aboveground dry matter (2.90 t ha−1), thus contributing the highest nitrogen (78.8 kg ha−1). Both local weeds and CC treatments produced considerably lower dry matter (0.87 and 0.59 t ha−1, respectively); therefore, their nitrogen contributions to the cropping system were minimal compared with SH and cover crop mixture (Figure 3). Interestingly, the local weeds in NC plots contained significantly more nitrogen (37.3 g kg−1) than in monoculture and mix legumes (Figure 2). The results indicate that the local weeds were more aggressive and efficient in taking up nitrogen from the soil. The effective nutrient uptake by weeds from the soil likely demonstrates their capacity to adapt to changes in edaphoclimatic conditions during the growing season [37,38]. To our surprise, CC performed poorly in terms of atmospheric nitrogen fixation and accumulation in plants' aerial tissues. The failure of CC to fix nitrogen could be due to the poor rhizobium activity and thus requires further investigation.

**Table 2.** Mean cover crop dry matter biomass and nitrogen content following different cover crops in South Deerfield MA in 2014–2015 and 2015–2016.


Note. \*, *p* ≤ 0.05; \*\*\*, *p* ≤ 0.001; ns, non-significant according to non-parametric permutation tests. For significant effects, all pairwise comparisons were made using Bonferroni adjusted *t*-tests. Means followed by the same letter are not significantly different from each other (*p* ≤ 0.05). Cover crop dry matter and nitrogen content includes weeds.
