@article {KNZ001100,
	title = {Seven years of enhanced water availability influences the physiological, structural and functional attributes of a soil microbial community},
	journal = {Applied Soil Ecology},
	volume = {35},
	year = {2007},
	pages = {535 -545},
	abstract = {Water availability is known to influence many aspects of microbial growth and physiology, but less is known about how complex soil microbial communities respond to changing water status. To understand how long-term enhancement of soil water availability (without flooding) influences microbial communities, we measured the seasonal dynamics of several community-level traits following >7 years of irrigation in a drought-prone tallgrass prairie soil. From late May to mid-September, water was supplied to the irrigated treatments based on calculated plant water demand. Phospholipid fatty acids (PLFA) were used to assess changes in microbial community structure and physiology. To assess the community-level physiological profile, microbial utilization of BIOLOG substrates was determined. After incubation for 2 days, the distribution of added 13C-glucose in microbial and respired pools was used as an index of substrate utilization efficiency. We also measured the relative contribution of fungi and bacteria to soil microbial biomass via substrate-induced respiration (SIR). Multivariate analysis of mol\% PLFA and BIOLOG substrate utilization indicated that both water availability and sampling time influenced both the physiological and structural characteristics of the soil microbial community. Specific change in biomarker PLFA revealed a decreased ratio of cyclopropyl to ω7-precursors due to water addition, suggesting community-level stresses were reduced. Over the growing season, continuously greater water availability resulted in a 53\% greater ratio of fungal to bacterial biomass using SIR, and a 65\% increase in fungal PLFA. The number of substrates utilized by the cultivable microbial community tended to be greater in continuously wetted soil, especially during periods of low rainfall. While water dynamics appeared to be associated with some of the shifts in microbial community activity, structural and functional changes in the community appeared to be more closely linked to the cumulative effects of water regime on ecosystem properties. Seasonality strongly influenced microbial communities. The environmental factors associated with seasonal change need to be more closely probed to better understand the drivers of community structure and function.},
	keywords = {LTER-KNZ, Microbial community, physiology, PLFA, Stress, Substrate use efficiency, Water availability},
	doi = {10.1016/j.apsoil.2006.09.014},
	author = {Williams, M.A. and C. W. Rice}
}
@article {KNZ00694,
	title = {Nitrogen limitation in dryland ecosystems: responses to temporal and geographical variation in precipitation},
	journal = {Biogeochemistry},
	volume = {46},
	year = {1999},
	pages = {247 -293},
	abstract = {We investigated the relationship between plant nitrogen limitation and water availability in dryland ecosystems. We tested the hypothesis that at lower levels of annual precipitation, aboveground net primary productivity (ANPP) is limited primarily by water whereas at higher levels of precipitation, it is limited primarily by nitrogen. Using a literature survey of fertilization experiments in arid, semi-arid, and subhumid ecosystems, we investigated the response of ANPP to nitrogen addition as a function of variation in precipitation across geographic gradients, as well as across year-to-year variation in precipitation within sites. We used four different indices to assess the degree of N limitation: (1) Absolute Increase of plant production in response to fertilization (the slope of ANPP vs. amount of added N at different levels of annual precipitation); (2) Relative Response (the percent increase in fertilized over control ANPP at different levels of N addition); (3) Fertilizer Use Efficiency (FUE, the absolute gain in productivity per amount of fertilizer N), and (4) Maximum Response (the greatest absolute increase in ANPP at saturating levels of N addition). Relative Response to fertilization did not significantly increase with increasing precipitation either across the geographic gradient or across year-to-year variation within sites. Nor did the Maximum Response to fertilization increase with increasing precipitation across the geographic gradient. On the other hand, there was a significant increase in the Absolute Increase and FUE indices with both geographical and temporal variation in precipitation. Together, these results indicate that there is not necessarily a shift of primary limitation from water to N across the geographic water availability gradient. Instead, our results support the hypothesis of co-limitation. The apparently contradictory results from the four indices of N limitation can best be explained by an integration of plant ecophysiological, community, and ecosystem mechanisms whereby plants are co-limited by multiple resources, species shifts occur in response to changing resource levels, and nitrogen and water availability are tightly linked through biogeochemical feedbacks.},
	keywords = {LTER-KNZ, arid, Fertilization, Net primary productivity, nitrogen limitation, semi-arid, Water availability},
	doi = {10.1007/BF01007582},
	author = {Hooper, D.U. and Johnson, L.C.}
}
