Use of biological indicators of soil health to assess N fertility and land management in annual and perennial organic agroecosystems

Background/Question/Methods

Organic agricultural systems are dependent upon fertilizer amendments that undergo ammonification prior to the release of plant available nitrogen (N) as ammonium (NH₄⁺). Ammonia may be further transformed via nitrification to nitrate resulting in greater potential loss of reactive N. Additions of plant residue and animal amendments contribute to soil N and carbon pools improving soil health and the potential for release of NH₄⁺. 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 losses of reactive N. Our experimental design allowed us to determine the effects of several organic cropping systems (annual vs perennial) and N fertility regimes (manure vs. compost) on plant available N, nitrification potential and nitrifier gene copies g^-1 soil across season (March, June, September). Nitrification potentials measured via the shaken slurry method, KCl extractable N and ammonia oxidizing bacteria (AOB) and archaea (AOA) gene copies g^-1 soil were measured via quantitative PCR.

Results/Conclusions

Nitrification potentials in March were highest in managements receiving compost (7.78 vs. 5.26 ± 0.856). Treatment affects were significant in June after application of amendments. Animal amendments were added yearly in annual systems (31.0 ± 2.91) vs every three years in the perennial (12.9 ± 2.91) resulting in the June management effect. Gene copies of AOB (2.69 x 10⁸ ± 4.94 x 10⁷) were highest were compost was applied. Gene copies of AOA amoA (10⁵ to 10⁶) were lower than AOB. Nitrification potentials did not differ in September (11.8 ± 2.28). Inorganic N correlated with nitrification potentials but not AOB or AOA amoA gene copies. Inorganic N and nitrification potentials reflected the addition of N via long-term compost applications. Green manures increased nitrification potentials and AOB amoA gene copies that served as sensitive indicators of N fertility and soil health in these perennial and annual organic systems. Ammonia oxidizing archaea, amoA gene copies were negatively correlated with nitrification and were insensitive to N additions. Our research suggests that the two nitrifier communities overlap but occupy different niches. This research will contribute to the development of novel cropping systems and land-use managements that maintain and promote efficient N cycling.