Accounting for variability in soil microbial communities of temperate upland grassland ecosystems

This study aimed to determine the factors which regulate soil microbial com munity organisation and function in temperate upland grassland ecosystems. Soil microbial biomass (C-mic), activity (respiration and potential carbon utilisation) and community structure (phospholipid fatty acid (PLFA) analys is, culturing and community level physiological profiles (CLPP) (Biolog*)) were measured across a gradient of three upland grassland types; Festuca-Ag rostis-Galium grassland (unimproved grassland, National Vegetation Classifi cation (NVC) - U4a); Festuca-Agrostis-Galium grassland, Holcus-Trifolium su b-community (semi-improved grassland, NVC - U4b): Lolium-Cynosurus grasslan d (improved grassland, NVC - MG6) at three sites in different biogeographic areas of the UK over a period of 1 year. Variation in C-mic. was mainly du e to grassland type and site (accounting for 55% variance, v, in the data). C-mic was significantly (P < 0.001) high in the unimproved grassland at To rridon (237.4 g C m(-2) cf. 81.2 g C m(-2) in semi- and 63.8 g C m(-2) in i mproved grasslands) and Sourhope (114.6 g C m(-2) cf. in 44.8 g C m(-2) sem i- and 68.3 g C m(-2) in improved grasslands) and semi-improved grassland a t Abergwyngregyn (76.0 g C m(-2) cf. 41.7 g C m(-2) in un- and 58.3 g C m(- 2) in improved grasslands). C-mic showed little temporal variation (v = 3.7 %). Soil microbial activity, measured as basal respiration was also mainly affected by grassland type and site(n = 32%). In contrast to C-mic, respira tion was significantly(P < 0.001) high in the improved grassland at Sourhop e (263.4 h(-1)m(-2) cf. 79.61 h(-1)m(-2) in semi- and 203.9 1 h(-1)m(-2) un improved grasslands) and Abergwyngregyn (198.81h(-1)m(-2) cf. 173.71 h(-1)m (-2) in semi- and 88.21 h(-1)m(-2) unimproved grasslands). Microbial activi ty, measured as potential carbon utilisation, agreed with the respiration m easurements and was significantly (P < 0.001) high in the improved grasslan d at all three sites (A(590) 0.14 cf. 0.09 in semi- and 0.07 in unimproved grassland). However, date of sampling also had a significant (P < 0.001) im pact on C utilisation potential (v = 24.7%) with samples from April 1997 ha ving highest activity at all three sites. Variation in microbial community structure was due, predominantly, to grassland type (average v = 23.6% for bacterial and fungal numbers and PLFA) and date of sampling (average v = 39 .7% for bacterial and fungal numbers and PLFA). Numbers of culturable bacte ria and bacterial PLFA were significantly (P < 0.001) high in the improved grassland at all three sites. Fungal populations were significantly (P < 0. 01) high in the unimproved grassland at Sourhope and Abergwyngregyn. The re sults demonstrate a shift in soil microbial community structure from one fa vouring fungi to one favouring bacteria as grassland improvement increased. Numbers of bacteria and fungi were also significantly (P < 0.001) higher i n August than any other sampling date. Canonical variate analysis (CVA) of the carbon utilisation data significantly (P < 0.05) differentiated microbi al communities from the three grassland types. mainly due to greater utilis ation of sugars and citric acid in the improved grasslands compared to grea ter utilisation of carboxylic acids, phenolics and neutral amino acids in t he unimproved grasslands, possibly reflecting substrate availability in the se grasslands. Differences in C-mic, activity and community structure betwe en grassland types were robust over time. In addition, broad scale measures of microbial growth and activity (C-mic a nd respiration) showed little temporal variation compared to measures of so il microbial community structure, which varied quantitatively with respect to environmental variables (temperature, moisture) and plant productivity. hence substrate supply. (C) 2001 Elsevier Science Ltd. All rights reserved.