Managing Agroecosystems for Soil Microbial Carbon Use Efficiency: Ecological Unknowns, Potential Outcomes, and a Path Forward

Agricultural systems are increasingly managed for improving soil carbon (C) accumulation. However, there are limits to C returns in agricultural systems that constrain soil C accumulation capacity. Increasing the efficiency of how soil microbes process C is gaining interest as an important management strategy for increasing soil C and is a key feature of soil C dynamics in many new microbial-explicit models. A higher microbial C use efficiency (CUE) may increase C storage while reducing C system losses and is a fundamental trait affecting community assembly dynamics and nutrient cycling. However, the numerous ecological unknowns influencing CUE limit our ability to effectively manage CUE in agricultural soils for greater soil C storage. In this perspective, we consider three complex drivers of agroecosystem CUE that need to be resolved to develop effective C sequestration management practices in the future: (1) the environment as an individual trait moderator versus a filter, (2) microbial community competitive and faciliatory interactions, and (3) spatiotemporal dynamics through the soil profile and across the microbial lifecycle. We highlight ways that amendments, crop rotations, and tillage practices might affect microbial CUE conditions and the variable outcomes of these practices. We argue that to resolve some of the unknowns of CUE dynamics, we need to include more mechanistic, trait-based approaches that capitalize on advanced methods and innovative field research designs within an agroecosystem-specific context. By identifying the management-level determinants of CUE expression, we will be better positioned to optimize CUE to increase soil C storage in agricultural systems

A holistic framework integrating plant-microbe-mineral regulation of soil bioavailable nitrogen

Soil organic nitrogen (N) is a critical resource for plants and microbes, but the processes that govern its cycle are not well-described. To promote a holistic understanding of soil N dynamics, we need an integrated model that links soil organic matter (SOM) cycling to bioavailable N in both unmanaged and managed landscapes, including agroecosystems. We present a framework that unifies recent conceptual advances in our understanding of three critical steps in bioavailable N cycling: organic N (ON) depolymerization and solubilization; bioavailable N sorption and desorption on mineral surfaces; and microbial ON turnover including assimilation, mineralization, and the recycling of microbial products. Consideration of the balance between these processes provides insight into the sources, sinks, and flux rates of bioavailable N. By accounting for interactions among the biological, physical, and chemical controls over ON and its availability to plants and microbes, our conceptual model unifies complex mechanisms of ON transformation in a concrete conceptual framework that is amenable to experimental testing and translates into ideas for new management practices. This framework will allow researchers and practitioners to use common measurements of particulate organic matter (POM) and mineral-associated organic matter (MAOM) to design strategic organic N-cycle interventions that optimize ecosystem productivity and minimize environmental N loss

Expansion of Agriculture in Northern Cold-Climate Regions: A Cross-Sectoral Perspective on Opportunities and Challenges

Agriculture in the boreal and Arctic regions is perceived as marginal, low intensity and inadequate to satisfy the needs of local communities, but another perspective is that northern agriculture has untapped potential to increase the local supply of food and even contribute to the global food system. Policies across northern jurisdictions target the expansion and intensification of agriculture, contextualized for the diverse social settings and market foci in the north. However, the rapid pace of climate change means that traditional methods of adapting cropping systems and developing infrastructure and regulations for this region cannot keep up with climate change impacts. Moreover, the anticipated conversion of northern cold-climate natural lands to agriculture risks a loss of up to 76% of the carbon stored in vegetation and soils, leading to further environmental impacts. The sustainable development of northern agriculture requires local solutions supported by locally relevant policies. There is an obvious need for the rapid development of a transdisciplinary, cross-jurisdictional, long-term knowledge development, and dissemination program to best serve food needs and an agricultural economy in the boreal and Arctic regions while minimizing the risks to global climate, northern ecosystems and communities

Root traits and root biomass allocation impact how wheat genotypes respond to organic amendments and earthworms

<div><p>Plant-soil biological interactions are increasingly recognized as a key feature of agroecosystems, promoting both crop and soil health. However, the effectiveness of plant-soil synergies is likely modulated by both root system characteristics and soil management impacts on soil biological communities. To successfully manage for plant-soil interactions, we need to better understand how crops respond to changes in soil management, especially in terms of belowground investment. Specifically, crop genotypes that exhibit reduced plasticity in root growth and investment may not be able to take full advantage of changes in soil biological activity associated with soil health promoting practices. We hypothesized that genotypes with greater belowground investment respond more, in terms of plant growth and crop nitrogen (N) uptake, to compost and earthworm additions, agronomic factors commonly associated with soil health. We evaluated four spring wheat (<i>Triticum aestivum</i>) genotypes with distinct breeding and environmental histories, and one progenitor of wheat (<i>Aegilops tauschii</i>) under low soil fertility conditions in the greenhouse for differences in belowground root biomass and architecture. We then determined how these belowground traits influenced genotype response to additions of compost and earthworms. Measurements included plant growth, biomass, grain yield, root characteristics, plant N uptake, and soil N. Overall, in unamended soils, genotypes differed in above and belowground phenotypic traits. In general, <i>Ae</i>. <i>tauschii</i> had three times greater root: shoot (R:S) ratio, root length, and root biomass relative to wheat genotypes. We found that genotypes with higher R:S ratios responded more positively to compost additions compared to those with lower R:S ratios, particularly in terms of plant aboveground biomass, N uptake and soil N-cycling, and also exhibited greater plasticity in root morphology. Consequently, while higher R:S genotypes had relatively poorer yields in unamended soils, they outperformed lower R:S genotypes in total seed weight under compost treatments. Our findings suggest that genotypes with greater belowground investment may be better able to take advantage of soil health promoting practices, such as the use of organic amendments. These results highlight the need to consider soil management practices (and associated biological communities) in parallel with root phenotypic plasticity when evaluating wheat lines for improvements in plant-soil synergies.</p></div

Genotypic response of aboveground growth to compost relative to control treatments without compost or earthworms.

<p>(A) aboveground biomass, (B) yield, (C) harvest index, (D) seed number, (E) grain N uptake, (F) and total aboveground N uptake. The relative change is: mean value with compost–mean value of no compost / mean value of no compost value. Genotype responses are shown in relationship to their R:S in unamended control soils. A significant response to compost (p <0.05) is indicated by an asterisk (*); <i>n</i> = 5).</p

Means and ANOVA results for aboveground production variables for <i>Ae</i>. <i>tauschii</i> and four spring wheat genotypes grown in the greenhouse under differing soil treatments.

<p>Standard errors are presented to the right of each mean.</p

Genotypic response of root growth to compost relative to control treatments without compost or earthworms.

<p>(A) root biomass, (B) root: shoot (R:S), (C) root length, (D) average root diameter, (E) root surface area, (F) and root angle. The relative change is: mean value with compost–mean value of no compost / mean value of no compost value. Genotype responses are shown in relationship to their R:S in unamended control soils. A significant response to compost (p <0.05) is indicated by an asterisk (*); <i>n</i> = 5).</p