Increased proportion of active soil N in Breton loam under cropping systems with forages and green manures

Total soil N and N mineralization\udrate partially ctririct&iie the influence olvariouJcropping systems on the growth of.sequent.crop.s in a rotation' The objectives\udof this study were to (1) quantify the relationship among cropping system, total N and mineralizable N, and (2) compare amount\udof N mineralized under'controlled laboratory conditions-with pianl N uptake under green]ouse conditi:ns. Three cropping systems\udthat have been in operation between 9 and 60 yr on a Gray-Luvisol (Breton loam; were\ud^selected . They included: (1) an ag.lo:\udecological (lnn) d-yr rotation involving fabibeans as gr-een manure (AER1 sampled after the_first lababean crop-?nq AER?\ud,u-pied after 3 yr of continuous forage)l (2) continuous grain system (CG), with fertilizer N at 90 kg ha-'^yr- '; inttrated\udin 1980 and considered established in f"98i; i3) a classicial Ereton iotation (CBR) involving 9]o-ng-term (ca. 1930) 5-yr rotation\udwith forages and cereals and no return of.ciop residues (CBR1 fertilized with P-K-S and CBR2 unfertilized). We cautio_n that\udnot all ohlses of each rotation were sampled: bur conclusions pertain to N-mineralization potential in soil samples immediately\udpreceding barley as sequent crop in each rotation. The rate ofN mineralization declined with time, but it remained greater than\udiero aftei 20 wi< of incirbation in all soils. Mineral-N accumulation at 20 wk followed the order AER1 > AER2 > > CBRI >\udCBR2 : CG. Mineralizable soil N, following one cycle of the AER rotation, was almost double that following 60 yr of the\udCBR rotation. Data for mineral-N accumulation under laboratory conditions were described best by a single-component expo-n-ential\udmodel. Legume-based rotations were associated with increased total soil N and a greater proportional increase in active N than\udin total soilN. Active N was least in soil under the CG system. The incubation-extraction procedure resulted in higher estimates\udof mineralizable N than did the plant-uptake method; liowever, the ranking of N-supplying power of soils was the same