Microbial Colonization of Legume Crop Residues During Decomposition in a Cold Temperate Climate.

The rotational benefits of pulse crops include N contribution to nonlegume crops.  The N is released from crop residues during microbial decomposition.  We quantified soil microbial biomass C (MBC) and bacterial diversity (Shannon index, H’) on decomposing residues of two pea varieties (Camry, a semi-leafless variety, and 4010, a normal-leafed silage pea variety), faba bean grown for seed, faba bean green manure (GM) and chickling vetch GM.  Nitrogen released from the residues was concurrently quantified.  During 12 months of decomposition, the order of MBC on residues was faba bean GM (57 g kg^-1 residue DM) ≥ vetch GM (51 g kg^-1 residue DM) ≥ faba bean grown for seed (48 g kg^-1 residue DM) ≥ Camry pea (43 g kg^-1 residue DM) ≥ 4010 pea (36 g kg^-1 residue DM), and H’ on Camry pea residues (2.68) was less than that on the other residues (2.81 to 2.89).  The pattern with time was a rapid increase of MBC in the first four weeks after residue placement at legume crop harvest in summer or fall, followed by a slight decrease during winter, then a gradual increase from spring to fall of the following season.  The pattern for H’ was similar, except that the decrease during winter was accelerated between spring and summer of the following season, and started increasing again between summer and fall.  The pattern of N release from the crop residues also followed a similar trend to that of MBC on the residues, except that N release continued to increase slowly during winter.  There was positive correlation (r = 0.413*, n = 30) between MBC on the residues and N released from the residues during the 12-month period even though faba bean grown for seed released the most N.  Bacterial diversity (H’) was not related to N released, presumably because H’ decreased between spring and summer when N release continued to increase.  H’ was negatively correlated with the residue C/N rations during the 12 months of decomposition (r = -0.367*, n = 30) even though the ratios did not change much, ranging between 9 and 13 for GM, 13 and 17 for faba bean grown for seed, and 41 and 47 for peas.  When only the first month of decomposition and N release were considered, both MBC and H’ were negatively correlated with %N remaining in residues (r = -0.535* and -0.592*, respectively, n = 15).  Therefore, microbial colonization of crop residues can be a good indicator of N released from the residues.