Submitted to: Biology and Fertility of Soils
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/24/2013
Publication Date: 3/14/2014
Citation: Dao, T.H. 2014. Landscape-scale geographic variations in microbial indices and labile phosphorus in Hapludults. Biology and Fertility of Soils. 50(1):155-167. Interpretive Summary: Phosphorus is an essential nutrient in the optimal growth and development of crop plants for the production of food and fiber crops for human and livestock consumption. Most natural and cultivated are deficient in P and require P inputs for optimal productivity. In the traditional mixed animal and plant production systems, manure-borne nutrients have been recycled in crop production. Soil and nutrient management practices to promote crop growth can have long-lasting effects on the distribution of soil microorganisms, the nutrient cycling processes they mediate, and the re-distribution of nutrients. This is particularly important to phosphorus that is a relatively non-mobile nutrient across a field. This non-random redistribution may influence the efficiency of crops to utilize the added nutrients, in comparison to mobile nutrients such as nitrate. The variability across a field is partly introduced during land application of agricultural by-products such as animal manure and composts. We determined the distribution of soil microbial biomass and selected enzymes involved in nutrient cycling in a field under a long-term corn-winter wheat rotation and land application of dairy manure. The research showed that a high biogeographic variability existed across a large field where there should not be any difference because of uniform rate of dairy manure applications. High microbial populations and enzyme activities in specific locations reflected a clustering of high rate of nutrient transformations, and mechanistic differences for the accumulation of labile organic P across the field. Where there were high levels of inorganic phosphate on the field resulting from uneven spreading of manure, there was suppressed soil enzyme activity involved in the breakdown of organic forms of P. Yet, conventional soil testing procedures do not adequately reflect the net accumulation or turnover of these organic P forms. Current soil testing methods for estimating biochemical or microbial parameters, labile organic P, and P requirements that are based on measurements of representative samples obtained by combining a large number of samples obtained from various locations on the field, should be avoided and substituted with site-specific sampling procedures to improve manure nutrient management and recommendation for beneficial re-uses.
Technical Abstract: Long-term soil and nutrient management practices can have lasting effects on the geographic distribution of soil microorganisms, function, and non-mobile nutrients such as phosphorus (P). The non-random redistribution can influence nutrient turnover rate and use efficiency of crops, in comparison to mobile nutrients such as NO3-. Spatial heterogeneity may be introduced during land applications of organic by-products such as animal manure and composts. We determined the spatial distribution of soil microbial biomass, phosphohydrolases, and bioactive P forms in a 10-ha field under a corn-winter wheat sequence. The spatial structure of soil microbial indices and extractable P fractions were characterized based on their semi-variance distributions. A high geographic variability in biomass C and enzyme activities existed, with clusters found in specific locations of the field. The clustering pattern reflected a localized high rate of nutrient turnover, and were apparently associated with the pattern of enzyme-hydrolyzable organic P accumulation. Suppressed phosphohydrolases' activities were also found in areas of the field where there was a high inorganic-to-organic P concentration ratio. The observed geographic heterogeneity highlighted the need to improve manure nutrient application practices to reduce management-induced variability, in particular for manure-borne non-mobile nutrients. Current soil testing methods for estimating biochemical or microbial parameters, labile organic P, and P requirements that are based on measurements of representative samples upon compositing a number of samples obtained from various locations on the field may need to be substituted with site-specific sampling approaches.