Effects of green manures on soil organic matter and wheat yields and N nutrition

A field study was conducted for 5 yr (1993-1997) to evaluate the effects of green manure residues applied to the soil in 1993 and 1995, on wheat (Trit icum aestivum L.) yields and N nutrition as subsequent crop in 1994, 1996 a nd 1997. The effect of green manure application was also evaluated on soil microbial activity (CO2), on C and N contents of whole soil and on labile ( LF) and heavy fractions (HF) of organic matter (On. The experiment was init iated on a Le Bras silt loam (Humic Gleysol). The green manures, as a main factor, were clover (Trifolium pratense L.), buckwheat (Fagapyrum esculentu m L.), millet (Echinicloa crus galli L.), mustard (Brassica hirta Moench), and colza (Brassica campestris L.), and there was a control without green m anure. The sub-factors consisted of four N fertilizer rates for wheat in th e subsequent years at 0, 30, 60 and 90 kg N ha(-1). Broadcast application o f N-15- labelled NH4NO3, was made in 90 kg N ha(-1) fertilizer treatments. Two green manure applications did not influence the C and N contents of den simetric fractions of OM (LF and HF), but significantly increased those of whole soil, and microbial respiration (CO2). Green manures significantly in creased wheat yields and N uptake in 1994 and 1996. Levels of N derived fro m fertilizer (Ndff) were lower in all green manure treatments as compared t o the control, which indicates that the proportion of N derived from soil a nd green manures (% Ndfs) was higher in these treatments. The contribution of N from green manure varied in the following order: buckwheat < clover < mustard < millet < colza. With the exception of the clover treatment (< 100 %), the N recoveries from the other green manure N (NRGM) ranged from 23 to 34% and from 19 to 36 % for 1994 and 1996, respectively. Green manure appl ication provided 15 to 24 kg N ha(-1) in 1994 and from 16 to 36 kg N ha(-1) in 1996 and this contribution accounted for 25 to 31 % of the total wheat N uptake. Significant green manure effects on wheat yield and N nutrition w ere primarily due to the improvement of soil properties and to high N recov eries from the green manure. In the cold temperate climate of Quebec, green manure incorporation into soil in late summer or early fall of the precedi ng year allowed N synchronization with wheat N needs in subsequent cropping seasons. Nitrogen fertilizer rates could be reduced after the incorporatio n of green manures having high yields and N contents in the previous season .