Soil microbial community responses to dairy manure or ammonium nitrate applications

Abstract

Soil management practices that result in increased soil C also impact soil microbial biomass and community structure. In this study, the effects of dairy manure applications and inorganic N fertilizer on microbial biomass and microbial community composition were determined. Treatments examined were a control with no nutrient additions (CT), ammonium nitrate at 218 kg N ha⁻¹ (AN), and manure N rates of 252 kg manure-N ha⁻¹ (LM) and 504 kg manure-N ha⁻¹ (HM). All plots were no-till cropped to silage corn (Zea mays, L. Merr) followed by a Crimson clover (Trifolium incarnatum, L.)/annual ryegrass (Lolium multiflorum, Lam.) winter cover crop. Treatments were applied yearly, with two-thirds of the N applied in late April or early May, and the remainder applied in September. Soil samples (0–5, 5–10, and 10–15 cm) were taken in March 1996, prior to the spring nutrient application. Polar lipid fatty acid (PLFA) analysis was used to assess changes in microbial biomass and community structure. Significantly greater soil C, N and microbial biomass in the 0–5 cm depth were observed under both manure treatments than in the CT and AN treatments. There was also a definable shift in the microbial community composition of the surface soils (0–5cm). Typical Gram-negative bacteria PLFA biomarkers were 15 and 27% higher in the LM and HM treatments than in the control. The AN treatment resulted in a 15% decrease in these PLFA compared with the control. Factor analysis of the polar lipid fatty acid profiles from all treatments revealed that the two manure amendments were correlated and could be described by a single factor comprised of typical Gram-negative bacterial biomarkers. The AN treatments from all three depths were also correlated and were described by a second factor comprised of typical Gram-positive bacterial biomarkers. These results demonstrate that soil management practices, such as manuring, that result in accumulations of organic carbon will result in increased microbial biomass and changes in community structure.

Introduction

It has been well documented that soil management can significantly impact soil biological and biochemical properties. Doran (1980) observed that soils in no-till cropping systems have higher microbial populations, largely due to increased soil organic carbon, than soils under tillage. He also observed evidence for a shift in microbial community structure, with higher counts of denitrifiers and facultative anaerobes. A North Carolina study (Kirchner et al., 1993) evaluated the impacts of a crimson clover (Trifolium incarnatum L.) cover crop in a conventionally-tilled continuous corn system. Microbial biomass carbon was higher under clover than under a winter fallow system. Heterotrophic bacteria numbers were generally higher in the clover system as well. The clover cover crop also resulted in higher activities of alkaline phosphatase, arylsulfatase and β-glucosidase. Mullen et al. (1998) observed that cover crops significantly increased several enzyme activities relative to no cover in no-till corn. The application of cattle manure slurry has been demonstrated to increase soil microbial biomass (Acea and Carballas, 1988, Ritz et al., 1997). Applications of dairy-feedlot manure have also been shown to maintain or increase soil carbon and microbial biomass to a greater extent than that realized with chemical fertilization (Dormaar et al., 1988). Management of soil that leaves residue on the soil surface, such as in no-tillage, often results in higher concentrations of soluble organic carbon compounds (Alvarez et al., 1998), which may result in the enhancement of microbial properties.

One problem with characterizing changes in soil microbial communities has been the insensitivity associated with many methods. Standard plate count methods may recover only a few percent of the organisms present, with those isolated restricted to organisms suited to growth on the medium of choice. Measurements of soil microbial biomass give an indication of the standing crop of microbial life in the soil, but provide no indication of community structure. In short, traditional methodologies of microbiology lack the ability to quantitatively and comprehensively describe diverse communities, such as in soil. The application of molecular techniques to the study of microorganisms in situ overcomes a number of these deficiencies providing a better understanding of how bacterial mediated processes are affected by changing management practices. One such method is the measurement of phospholipid fatty acids (PLFA) in the soil.

Phospholipid lipid fatty acids can be used to describe viable microbial communities in terms of total biomass (Balkwill et al., 1988) and community composition (Vestal and White, 1989). Phospholipids are essential components of life making up the bulk of the matter of cell membranes. Since phospholipids are rapidly degraded by endogenous and exogenous phospholipases upon cell death, they are reliable measures of viable cell biomass (White et al., 1979). The extraction and purification methods used are optimized for phospholipid molecules, so ‘fossil’ fatty acids, such as free fatty acids or those attached to humic substances, are not detected. PLFA have also been shown to be sufficiently distinct to allow for the identification of individual species of bacteria (Guckert et al., 1991) as well as provide insight into the make-up of whole communities (Frostegård et al., 1993). Community composition can be inferred from a PLFA profile by relating the type of PLFA identified to different biosynthetic pathways utilized in fatty acid synthesis (Fredrickson et al., 1995) or to specific species or classes of bacteria which contain a unique signature PLFA biomarker (Vestal and White, 1989).

This study was part of a larger experiment to evaluate impacts of manure applications on surface and subsurface water quality and soil properties. Based upon past work and literature review, we hypothesized that manuring would increase the microbial biomass relative to the control and ammonium nitrate treatments, and cause shifts in the microbial community structure. Therefore, the objective of this study was to document the effect of several years of dairy manure applications on microbial biomass and community structure, using PLFA methods, in a no-till corn silage cropping system.