Maintaining multiple ecological functions ("multifunctionality") is crucial to sustain viable ecosystems. To date most studies on biodiversity-ecosystem functioning (BEF) have focused on single or few ecological functions and services. However, there is a critical need to evaluate how species and species assemblages affect multiple processes at the same time, and how these functions are interconnected. Dung beetles represent excellent model organisms because they are key contributors to several ecosystem functions. Using a novel method based on the application of N-enriched dung in a mesocosm field experiment, we assessed the role of dung beetles in regulating multiple aspects of nutrient cycling in alpine pastures over appropriate spatial (up to a soil depth of 20 cm) and temporal (up to 1 yr after dung application) scales. N isotope tracing allowed the evaluation of multiple interrelated ecosystem functions responsible for the cycling of dung-derived nitrogen (DDN) in the soil and vegetation. We also resolved the role of functional group identity and the importance of interactions among co-occurring species for sustaining multiple functions by focusing on two different dung beetle nesting strategies (tunnelers and dwellers). Species interactions were studied by contrasting mixed-species to single-species assemblages, and asking whether the former performed multiple functions better than the latter. Dung beetles influenced at least seven ecological functions by facilitating dung removal, transport of DDN into the soil, microbial ammonification and nitrification processes, uptake of DDN by plants, herbage growth, and changes in botanical composition. Tunnelers and dwellers were found to be similarly efficient for most functions, with differences based on the spatial and temporal scales over which the functions operated. Although mixed-species assemblages seemed to perform better than single-species, this outcome may be dependent on the context. Most importantly though, the different functions were found to be interconnected sequentially as reveled by analyzing N content in dung, soil and vegetation. Taken together, our current findings offer strong support for the contention that the link between biodiversity and ecosystem functions should be examined not function by function, but in terms of understanding multiple functions and how they interact with each other.
Abstract. Soil moisture strongly affects the balance between nitrification, denitrification
and N2O reduction and therefore the nitrogen (N) efficiency and N
losses in agricultural systems. In rice systems, there is a need to improve
alternative water management practices, which are designed to save water and
reduce methane emissions but may increase N2O and decrease nitrogen
use efficiency. In a field experiment with three water management treatments,
we measured N2O
isotope ratios of emitted and pore air N2O
(δ15N, δ18O and site preference, SP) over the
course of 6 weeks in the early rice growing season. Isotope ratio
measurements were coupled with simultaneous measurements of pore water
NO3-, NH4+, dissolved organic carbon (DOC), water-filled pore space (WFPS) and soil redox potential (Eh) at three soil depths.
We then used the relationship between SP × δ18O-N2O and
SP × δ15N-N2O in simple two end-member
mixing models to evaluate the contribution of nitrification, denitrification
and fungal denitrification to total N2O emissions and to estimate
N2O reduction rates. N2O emissions were higher in a
dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to
water-seeded + alternate wetting and drying (WS-AWD) and
water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD
treatment the highest emissions were associated with a high contribution from
denitrification and a decrease in N2O reduction, while in the WS
treatments, the highest emissions occurred when contributions from
denitrification/nitrifier denitrification and nitrification/fungal
denitrification were more equal. Modeled denitrification rates appeared to be
tightly linked to nitrification and NO3- availability in all
treatments; thus, water management affected the rate of denitrification and
N2O reduction by controlling the substrate availability for each
process (NO3- and N2O), likely through changes in
mineralization and nitrification rates. Our model estimates of mean
N2O reduction rates match well those observed in 15N
fertilizer labeling studies in rice systems and show promise for the use of
dual isotope ratio mixing models to estimate N2 losses.
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