Plant–microbe interactions and nitrogen dynamics during wetland establishment in a desert stream
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In late-successional steady state ecosystems, plants and microbes compete for nutrients and nutrient retention efficiency is expected to decline when inputs exceed biotic demand. In carbon (C)-poor environments typical of early primary succession, nitrogen (N) uptake by C-limited microbes may be limited by inputs of detritus and exudates derived from contemporaneous plant production. If plants are N-limited in these environments, then this differential limitation may lead to positive relationships between N inputs and N retention efficiency. Further, the mechanisms of N removal may vary as a function of inputs if plant-derived C promotes denitrification. These hypotheses were tested using field surveys and greenhouse microcosms simulating the colonization of desert stream channel sediments by herbaceous vegetation. In field surveys of wetland (ciénega) and gravelbed habitat, plant biomass was positively correlated with nitrate (NO3 −) concentration. Manipulation of NO3 − in flow-through microcosms produced positive relationships among NO3 − supply, plant production, and tissue N content, and a negative relationship with root:shoot ratio. These results are consistent with N limitation of herbaceous vegetation in Sycamore Creek and suggest that N availability may influence transitions between and resilience of wetland and gravelbed stable states in desert streams. Increased biomass in high N treatments resulted in elevated rates of denitrification and shifts from co-limitation by C and NO3 − to limitation by NO3 − alone. Overall NO3 − retention efficiency and the relative importance of denitrification increased with increasing N inputs. Thus the coupling of plant growth and microbial processes in low C environments alters the relationship between N inputs and exports due to increased N removal under high input regimes that exceed assimilative demand.
KeywordsDenitrification Uptake Nutrient retention Ciénega Paspalum distichum Regime shift
We thank Sam Norlin and Margaret Gibson for their assistance with the maintenance of this experiment. Ryan Sponseller, Maury Valett, Jim Elser, and Ann Kinzig, as well as two anonymous reviewers, provided helpful comments that significantly improved this manuscript. This work was supported by a Doctoral Dissertation Improvement Grant (NSF DEB#07142) and a Dissertation Fellowship from the ASU Division of Graduate Studies to JBH. This research was carried out in accordance with the laws of the United States of America.
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