The Effect of Organic Cropping Systems, Fertility Source and Time of Season on Nitrification.

Organic agricultural systems are dependent upon fertilizer amendments that undergo ammonification prior to the release of plant available nitrogen (N) as ammonium. Ammonia may be further transformed via nitrification to nitrate resulting in greater potential for leaching or volatilization. Additions of plant residue and animal amendments contribute to soil N and carbon pools improving soil quality and the potential for release of ammonium. Therefore, agricultural systems that relay on organic inputs as fertilizer sources must be monitored to insure stored nutrients are released during plant uptake to prevent N losses. Our experimental design allows us to determine the effects of several organic cropping systems and fertility regimes on plant available N, nitrification potential and nitrifier gene copy number g^-1 dry soil across season in a grass ley and two annual systems receiving chicken manure or compost. Nitrification potentials measured via the shaken slurry method, KCl extractable N and ammonia oxidizing bacteria (AOB) and archaea (AOA) gene copy numbers g^-1 dry soil measured via quantitative PCR were monitored. Nitrification potentials measured in March revealed increases in nitrification where compost had historically been applied (7.78 vs. 5.26 ± 0.856). Treatment affects were significant in June the closest date to application of amendments and when ammonification from soil N was optimal. Animal amendments were added yearly in annual systems (31.0 ± 2.91) vs every three years in the ley pasture (12.9 ± 2.91) resulting in a management effect. Copy numbers of AOB (2.69 x 10⁸ ± 4.94 x 10⁷) were greatest in plots receiving compost vs chicken manure. Preliminary measurements of AOA gene copy numbers reveal a similar trend to that of AOB. But, gene copy numbers of AOA (10⁵ to 10⁶) were lower than those of AOB. Nitrification potentials were not different across treatments in September, 11.8 ± 2.28. This research will contribute to the development of novel cropping systems and land-use managements that maintain and promote efficient N cycling.