Intensive grassland management disrupts below-ground multi-trophic resource transfer in response to drought.

Modification of soil food webs by land management may alter the response of ecosystem processes to climate extremes, but empirical support is limited and the mechanisms involved remain unclear. Here we quantify how grassland management modifies the transfer of recent photosynthates and soil nitrogen through plants and soil food webs during a post-drought period in a controlled field experiment, using in situ 13C and 15N pulse-labelling in intensively and extensively managed fields. We show that intensive management decrease plant carbon (C) capture and its transfer through components of food webs and soil respiration compared to extensive management. We observe a legacy effect of drought on C transfer pathways mainly in intensively managed grasslands, by increasing plant C assimilation and 13C released as soil CO2 efflux but decreasing its transfer to roots, bacteria and Collembola. Our work provides insight into the interactive effects of grassland management and drought on C transfer pathways, and highlights that capture and rapid transfer of photosynthates through multi-trophic networks are key for maintaining grassland resistance to drought.

Soil organic carbon response to global environmental change depends on its distribution between mineral-associated and particulate organic matter: A meta-analysis.

Soil organic carbon (SOC), as the largest terrestrial carbon pool, plays an important role in global carbon (C) cycling, which may be significantly impacted by global changes such as nitrogen (N) fertilization, elevated carbon dioxide (CO2), warming, and increased precipitation. Yet, our ability to accurately detect and predict the impact of these global changes on SOC dynamics is still limited. Investigating SOC responses to global changes separately for mineral-associated organic carbon (MAOC) and the particulate organic carbon (POC) can aid in the understanding of overall SOC responses, because these are formed, protected, and lost through different pathways. To this end, we performed a systematic meta-analysis of the response of SOC, MAOC, and POC to global changes. POC was particularly responsive, confirming that it is a better diagnostic indicator of soil C changes in the short-term, compared to bulk SOC and MAOC. The effects of elevated CO2 and warming were subtle and evident only in the POC fraction (+5.11% and - 10.05%, respectively), while increased precipitation had no effects at all. Nitrogen fertilization, which comprised the majority of the dataset, increased SOC (+5.64%), MAOC (+4.49%), and POC (+13.17%). Effect size consistently varied with soil depth and experiment length, highlighting the importance of long-term experiments that sample the full soil profile in global change SOC studies. In addition, SOC pool responses to warming were modified by degree of warming, differently for air and soil warming manipulations. Overall, we suggest that MAOC and POC respond differently to global changes and moderators because of the different formation and loss processes that control these pools. Coupled with additional plant and microbial measurements, studying the individual responses of POC and MAOC improves understanding of the underlying dynamics of SOC responses to global change. This will help inform the role of SOC in mitigating the climate crisis.

Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century.

Managing soil organic matter (SOM) stocks to address global change challenges requires well-substantiated knowledge of SOM behavior that can be clearly communicated between scientists, management practitioners, and policy makers. However, SOM is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its dynamics. Numerous diverse SOM separation schemes are currently used, making cross-study comparisons difficult and hindering broad-scale generalizations. Here, we recommend separating SOM into particulate (POM) and mineral-associated (MAOM) forms, two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their highly contrasting physical and chemical properties, mean residence times in soil, and responses to land use change, plant litter inputs, warming, CO2 enrichment, and N fertilization. Conceptualizing SOM into POM versus MAOM is a feasible, well-supported, and useful framework that will allow scientists to move beyond studies of bulk SOM, but also use a consistent separation scheme across studies. Ultimately, we propose the POM versus MAOM framework as the best way forward to understand and predict broad-scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers.

Rethinking sources of nitrogen to cereal crops.

Understanding how to manage N inputs to identify the practices that maximize N recovery has been an organizing principle of agronomic research. Because growth in N fertilizer inputs is expected to continue in an ongoing effort to boost crop production over coming decades, understanding how to efficiently manage recovery of fertilizer N will be important going forward. Yet synthesis of published data that has traced the fate of 15 N-labeled fertilizer shows that less than half of the N taken up by crops is derived from current-year N fertilizer. The source of the majority of N in crops is something other than current-year fertilizer and the sources are not really known. This is true for maize (only 41% of N in crops was from current-year N fertilizer), rice (32%), and small grains (37%). Recovery of organic fertilizer N (manure, green manure, compost, etc.) in crops is low (27%), though N recovery in subsequent years (10%) was greater than that for mineral fertilizers. Thus, while research on efficiency of N fertilizer use through improved rate, type, location, and timing is important, this research fails to directly address management of the majority of the N supplied to crops. It seems likely that the majority of non-fertilizer N found in crops comes from turnover of soil and crop residue N. We encourage the research community to revisit the mental model that fertilizer is a replacement for N supply from turnover of soil organic N (SON) and consider a model in which N fertilizer augments ongoing SON turnover and makes an important longer term contribution to SON maintenance and turnover. Research focused on the efficient recovery of N current-year fertilizer inputs neglects this potential role for building soil N and managing soil N turnover, which seems likely to be the most important source of crop N.

Drought decreases incorporation of recent plant photosynthate into soil food webs regardless of their trophic complexity.

Theory suggests that more complex food webs promote stability and can buffer the effects of perturbations, such as drought, on soil organisms and ecosystem functions. Here, we tested experimentally how soil food web trophic complexity modulates the response to drought of soil functions related to carbon cycling and the capture and transfer below-ground of recent photosynthate by plants. We constructed experimental systems comprising soil communities with one, two or three trophic levels (microorganisms, detritivores and predators) and subjected them to drought. We investigated how food web trophic complexity in interaction with drought influenced litter decomposition, soil CO2 efflux, mycorrhizal colonization, fungal production, microbial communities and soil fauna biomass. Plants were pulse-labelled after the drought with 13 C-CO2 to quantify the capture of recent photosynthate and its transfer below-ground. Overall, our results show that drought and soil food web trophic complexity do not interact to affect soil functions and microbial community composition, but act independently, with an overall stronger effect of drought. After drought, the net uptake of 13 C by plants was reduced and its retention in plant biomass was greater, leading to a strong decrease in carbon transfer below-ground. Although food web trophic complexity influenced the biomass of Collembola and fungal hyphal length, 13 C enrichment and the net transfer of carbon from plant shoots to microbes and soil CO2 efflux were not affected significantly by varying the number of trophic groups. Our results indicate that drought has a strong effect on above-ground-below-ground linkages by reducing the flow of recent photosynthate. Our results emphasize the sensitivity of the critical pathway of recent photosynthate transfer from plants to soil organisms to a drought perturbation, and show that these effects may not be mitigated by the trophic complexity of soil communities, at least at the level manipulated in this experiment.