Global impacts of fertilization and herbivore removal on soil net nitrogen mineralization are modulated by local climate and soil properties

Authors: RISCH, A - Swiss Federal Research Institute Wsl; ZIMMERMANN, S - Swiss Federal Research Institute Wsl; MOSER, B - Swiss Federal Research Institute Wsl; SCHUTZ, M - Swiss Federal Research Institute Wsl; HAGEDORN, F - Swiss Federal Research Institute Wsl; FIRN, J - Queensland University Of Technology; Fay, Philip; ADLER, P - Utah State University; BIEDERMAN, L - Iowa State University; BLAIR, J - Kansas State University; BORER, E - University Of Minnesota; BROADBENT, A - University Of Manchester; CADOTTE, M - University Of Toronto; CALDEIRA, M - Universidade Nova De Lisboa; DAVIES, K - University Of Colorado; DI VIRGILIO, A - Consejo Nacional De Investigaciones Científicas Y Técnicas(CONICET); EISENHAUER, N - German Centre For Integrative Biodiversity Research (IDIV); ESKELINEN, A - University Of Oulu; KNOPS, J - Jiaotong University; MACDOUGALL, A - University Of Guelph; MCCULLEY, R - University Of Kentucky; MELBOURNE, B - University Of Colorado; MOORE, J - Monash University; POWER, S - Western Sydney University; PROBER, S - Commonwealth Scientific And Industrial Research Organisation (CSIRO); SEABLOOM, E - University Of Minnesota; SIEBERT, J - Leipzig University; SILVEIRA, M - University Of Florida; SPEZIALE, K - Consejo Nacional De Investigaciones Científicas Y Técnicas(CONICET); STEVENS, C - Lancaster University; TOGNETTI, P - Universidad De Buenos Aires; VIRTANEN, R - University Of Oulu; YADJIAN, L - Consejo Nacional De Investigaciones Científicas Y Técnicas(CONICET); OCHOA-HUESO, R - University Of Cadiz

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2020
Publication Date: 11/22/2020
Citation: Risch, A.C., Zimmermann, S., Moser, B., Schutz, M., Hagedorn, F., Firn, J., Fay, P.A., Adler, P.B., Biederman, L.A., Blair, J.M., Borer, E.T., Broadbent, A.A., Cadotte, M.W., Caldeira, M.C., Davies, K.F., Di Virgilio, A., Eisenhauer, N., Eskelinen, A., Knops, J.M., Macdougall, A.S., Mcculley, R.L., Melbourne, B.A., Moore, J.L., Power, S.A., Prober, S.M., Seabloom, E.W., Siebert, J., Silveira, M.L., Speziale, K.L., Stevens, C.J., Tognetti, P.M., Virtanen, R., Yadjian, L., Ochoa-Hueso, R. 2020. Global impacts of fertilization and herbivore removal on soil net nitrogen mineralization are modulated by local climate and soil properties. Global Change Biology. 26:7173-7185. https://doi.org/10.1111/gcb.153098.
DOI: https://doi.org/10.1111/gcb.153098

Interpretive Summary: Nitrogen is one of the key elements in soil determining the productivity of natural and managed grasslands. It’s availability to support plant growth, and ultimately to support native herbivores or domestic livestock, varies depending on numerous factors, but two main ones are soil texture and climate, which control the microbial decomposition and processing of soil organic matter to release nitrogen in plant-available forms. The scientific literature to date has not provided a clear picture of how climate and soils combine to determine the rate at which nitrogen is made available when considered on a global scale. This study addresses that gap, which is critical because excess nitrogen in the environment and losses of herbivore populations are dramatically and negatively impacting the terrestrial capacity to provide necessary ecosystem services. Using a unique dual-analysis approach, we show that adding nitrogen by fertilizing and excluding grazing mammals at many grassland sites across the globe will either raise or lower the rate at which nitrogen becomes available, depending on how sites varied in mean annual precipitation, clay content, temperature, and whether the nitrogen availability rate was measured directly in the field or in controlled laboratory conditions. The key implication of these findings is that in a nutrient-enriched, herbivore impoverished, and climatically variable world there may be negative consequences for nitrogen availability, but detecting these consequences depends critical on the approach used to measure it.

Technical Abstract: Soil nitrogen (N) availability is critical for grassland ecosystem functioning. However, human activities have increased the supply of biologically-limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin), the net balance between N mineralization and immobilization, which could severely impact the structure and functioning of grasslands. Yet, to date, little is known about how these global change drivers individually, or collectively, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here, we collected data from 22 natural and semi-natural grasslands on five continents, which are part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory-based) and realized (field-based) soil net Nmin. Potential soil net Nmin indicates the potential of grasslands to respond to global change, while realized soil net Nmin shows how well grasslands can withstand global change under realistic field conditions. We found that herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin. However, herbivore removal, in combination with fertilization, as well as fertilization alone, consistently increased potential soil net Nmin. Realized soil net Nmin, in contrast, significantly decreased on fertilized plots where also herbivores were removed. Site specific soil and climatic properties strongly mediated treatment effects on potential and realized soil net Nmin. In particular, mean annual precipitation and temperature of the wettest quarter altered the effects of fertilization and herbivore removal on N mineralization. In summary, our findings show that anthropogenic nutrient enrichment, changes in grazing regimes and climatic conditions strongly impact soil net Nmin across a wide range of grasslands under realistic field conditions. This has large implications for the sustainable management of grasslands worldwide.