Effects of Grazing on Microbial Functional Groups Involved in Soil N Dynamics

Abstract

Enhancement of soil nitrogen (N) cycling by grazing has been observed in many grassland ecosystems. However, whether grazing affects the activity only of the key microbial functional groups driving soil N dynamics or also affects the size (cell number) and/or composition of these groups remains largely unknown. We studied the enzyme activity, size, and composition of five soil microbial communities (total microbial and total bacterial communities, and three functional groups driving N dynamics: nitrifiers, denitrifiers, and free N2 fixers) in grassland sites experiencing contrasting sheep grazing regimes (one light grazing [LG] site and one intensive grazing [IG] site) at two topographical locations. Enzyme activity was determined by potential carbon mineralization, nitrification, denitrification, and N2 fixation assays. The size of each community (except N2 fixers) was measured by the most‐probable‐number technique. The composition of the total soil microbial community was characterized by phospholipid fatty acid analysis (PLFA), and the genetic structure of the total bacterial community was assessed by ribosomal intergenic spacer analysis. The genetic structures of the ammonia‐oxidizing, nitrate‐reducing, and N2‐fixing communities were characterized by polymerase chain reaction and restriction fragment length polymorphism (PCR‐RFLP) or by polymerase chain reaction and denaturing gradient gel electrophoresis (PCR‐DGGE) targeting group‐specific genes. Greater enzyme activities, particularly for nitrification, were observed in IG than in LG sites at both topographical locations. The numbers of heterotrophs, nitrifiers, and denitrifiers were higher in IG than in LG sites at both topographical locations. The amplitude of changes in community size was higher than that of community enzyme activity. Phospholipid and nucleic acid analyses showed that the composition/structure of all the communities, except nitrate reducers, differed between IG and LG sites at both locations. For each community, changes in activity were correlated with changes in the occurrence of a few individual PLFAs or DNA fragments. Our results thus indicate that grazing enhances the activity of soil microbial communities but also concurrently induces changes in the size and composition/structure of these communities on the sites studied. Although the generality of our conclusions should be tested in other systems, these results are of major importance for predicting the effects of future disturbances or changed grazing regimes on the functioning of grazed ecosystems.

Relationship between N-cycling communities and ecosystem functioning in a 50-year-old fertilization experiment

“…When looking at the functional communities, differences in potential denitrification rates were significantly correlated with differences in denitrifier density estimated by the quantification of nosZ gene copy numbers (Table 2). These results are consistent with recent studies showing that denitrification enzyme activity was correlated with the size of the denitrifier community estimated by most probable number (Patra et al, 2005) or by the quantification of nirK genes (Throbäck et al, 2007), which suggests that the size of the denitrifier community could predict the corresponding process rate. On the other hand, we did not find significant correlations between denitrification rates and the abundance of nirS and nirK genes.…”

Effects of aboveground grazing on coupling among nitrifier activity, abundance and community structure

“…However, the activity and abundance of nitrifiers were always higher on G-G than U-U soils and AOB community structure always differed between these treatments throughout the 24-month period studied. Previous analyses of grazing effects on AOB communities in grasslands were generally based on snapshot characterization at only one or two times (Webster et al, 2002;Patra et al, 2005). Comparison of G-G and U-U treatments show that grazing-induced enhancement of nitrifier activity and of both AOB and AOA abundances was remarkably conserved with time.…”

“…In particular, the regulation of nitrification has often been regarded as the key for an efficient nitrogen cycling in this context, as in contrast to most other steps in nitrogen turnover only a limited number of microbes is able to convert ammonia into nitrate. Enhanced nitrification in response to grazing pressure has been reported in different unfertilized grassland ecosystems (Groffman et al, 1993;Frank et al, 2000;Le Roux et al, 2003;Patra et al, 2005). These changes can be explained by factors such as urine and dung input by herbivores (Ruess and McNaughton, 1987;Petersen et al, 2004) and changes in competition for N between microorganisms and plants (Busso et al, 2001;Hamilton and Frank, 2001).…”

“…These changes can be explained by factors such as urine and dung input by herbivores (Ruess and McNaughton, 1987;Petersen et al, 2004) and changes in competition for N between microorganisms and plants (Busso et al, 2001;Hamilton and Frank, 2001). It has been further shown that grazinginduced differences in nitrification levels in grasslands are associated with changes in the abundance and community structure of ammonia-oxidizing bacteria (AOB) that were thought to be the key players of the first step of nitrification: oxidation of ammonia into nitrite (Patra et al, 2005(Patra et al, , 2006. This picture has changed a little in the last years, as it could be shown that ammonia oxidation can be performed not only by AOB but also by ammoniaoxidizing archaeae (AOA) that could be detected in soil (Leininger et al, 2006).…”

Lack of increased availability of root-derived C may explain the low N2O emission from low N-urine patches

“…To determine denitrifying and nitrifying enzyme activities, two soil cores (0-10 cm depth, 2 cm diameter) were collected in each monolith on day -1, 2 and 7 after the Denitrifying enzyme activity (DEA) was measured on the fresh soil samples over a short period according to Smith and Tiedje (1979) (for details, see Patra et al 2005). Briefly, 10 g (equivalent oven-dried) soil was placed into 150 ml flasks, and KNO 3 (200 µg NO 3 -N g -1 dry soil), glucose (0.5 mg C g -1 dry soil) and glutamic acid (0.5 mg C g -1 dry soil) was added.…”