Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/11/2013
Publication Date: 10/25/2013
Citation: Strauss, S.L., Reardon, C.L., Mazzola, M. 2013. The response of ammonia-oxidizer activity and community structure to fertilizer amendment of orchard soils. Soil Biology and Biochemistry. 68:410-418. Interpretive Summary: Nitrogen is one of the key nutrients required for plant growth, and the primary ingredient in most fertilizers. Soil microorganisms have the potential to dramatically alter the nitrogen (N) availability in agricultural systems, and therefore affect the efficiency of fertilizer application. Apple orchards require high inputs of N fertilizer, and organic and integrated management practices may enhance soil quality and profitability by influencing the soil microbial community. Studies examining the relationship between soil microbes in cereal-cropped systems of both organic and conventional managements and with different fertilizers have provided variable results. However, few assessments have been conducted on perennial crop production systems, including apple. As there are significant differences between cereal and apple root systems, and therefore the soil microbial community residing in those systems, there may be dramatic differences in the response of these microbes to organic management systems and alternative fertilizers. ARS scientists from Wenatchee, WA and Pendleton, OR, examined the abundance and activity of bacteria and archaea responsible for ammonia-oxidation, the first and rate-limiting step in nitrification, in organically and conventionally managed orchard soils amended with one of five fertilizer treatments. In both organically and conventionally managed orchard soils, the ammonia-oxidizing archaea (AOA) were more abundant than the ammonia-oxidizing bacteria (AOB). However, while AOA were more abundant, they had little response to the different soil management types and fertilizers. AOA were first identified 20 years ago and there is still little known about their physiology and growth, but they are suspected, and supported by these results, to be key to soil N cycling. AOB did respond to the soil management types and fertilizers, and indicated the conventional orchard soil may be limited by inorganic N. These significant differences in the microbial ammonia-oxidizer community between organic and conventional orchard soils highlight the need for an increased focus in agricultural research on understanding and improving the specificity of fertilizer application for orchard production systems.
Technical Abstract: Soil microorganisms have the potential to dramatically alter the nitrogen (N) availability in agricultural systems, and therefore affect the efficiency of fertilizer application. Data regarding the effects of cereal management systems on the soil microbiology functional to N cycling have yielded variable results. Few assessments concerning the influence of management practices on the structure and function of N cycling microbial communities have been conducted in perennial crop production systems, including apple (Malus domestica Borkh.). In addition, significant differences in the physiology of apple root systems compared to cereal crops may result in dramatic differences in the composition and activity of the soil microbial community, and thus the effective response of this population to organic management systems and alternative fertilizers. We examined the overall activity of ammonia-oxidizers and the abundance and diversity of bacterial and archaeal ammonia-oxidizing amoA genes in apple orchard soils amended with different organic fertilizers. Apple rootstocks were planted in organically and conventionally-managed orchard soils amended with one of five treatments: Brassica napus seed meal, B. napus seed meal with plant-based compost, plant-based compost, urea, urea with plant-based compost, and a no-treatment control. In both untreated conventionally and organically managed orchard soils, ammonia-oxidizing archaea (AOA) were more abundant than ammonia-oxidizing bacteria (AOB), but there was no significant difference in the microbial community composition of these untreated soils. However, there were significant differences in the AOB response to fertilizer treatments in both organically and conventionally managed soils. The microbial community of the conventional orchard soil appeared to be limited by inorganic N since a N response was only observed in treatments with urea. Comparatively, none of the fertilizer treatments produced an observable N response in the organic soil with the exception of increased AOB abundance with urea and compost. Soil management and fertilizer additions had little effect on AOA gene abundance, and no significant effect on the community composition of either AOB or AOA in both orchard soils. These significant differences in the microbial ammonia-oxidizer community between organic and conventional orchard soils highlight the need for an increased focus in agricultural research on understanding and improving the specificity of fertilizer application for orchard production systems.