Lisa K. Tiemann

Does agricultural crop diversity enhance soil microbial biomass and organic matter dynamics? A meta-analysis

Our increasing dependence on a small number of agricultural crops, such as corn, is leading to reductions in agricultural biodiversity. Reductions in the number of crops in rotation or the replacement of rotations by monocultures are responsible for this loss of biodiversity. The belowground implications of simplifying agricultural plant communities remain unresolved; however, agroecosystem sustainability will be severely compromised if reductions in biodiversity reduce soil C and N concentrations, alter microbial communities, and degrade soil ecosystem functions as reported in natural communities. We conducted a meta-analysis of 122 studies to examine crop rotation effects on total soil C and N concentrations, and the faster cycling microbial biomass C and N pools that play key roles in soil nutrient cycling and physical processes such as aggregate formation. We specifically examined how rotation crop type and management practices influence C and N dynamics in different climates and soil types. We found that adding one or more crops in rotation to a monoculture increased total soil C by 3.6% and total N by 5.3%, but when rotations included a cover crop (i.e., crops that are not harvested but produced to enrich the soil and capture inorganic N), total C increased by 8.5% and total N 12.8%. Rotations substantially increased the soil microbial biomass C (20.7%) and N (26.1%) pools, and these overwhelming effects on microbial biomass were not moderated by crop type or management practices. Crop rotations, especially those that include cover crops, sustain soil quality and productivity by enhancing soil C, N, and microbial biomass, making them a cornerstone for sustainable agroecosystems.

Crop rotational diversity enhances belowground communities and functions in an agroecosystem

Biodiversity loss, an important consequence of agricultural intensification, can lead to reductions in agroecosystem functions and services. Increasing crop diversity through rotation may alleviate these negative consequences by restoring positive aboveground-belowground interactions. Positive impacts of aboveground biodiversity on belowground communities and processes have primarily been observed in natural systems. Here, we test for the effects of increased diversity in an agroecosystem, where plant diversity is increased over time through crop rotation. As crop diversity increased from one to five species, distinct soil microbial communities were related to increases in soil aggregation, organic carbon, total nitrogen, microbial activity and decreases in the carbon-to-nitrogen acquiring enzyme activity ratio. This study indicates positive biodiversity-function relationships in agroecosystems, driven by interactions between rotational and microbial diversity. By increasing the quantity, quality and chemical diversity of residues, high diversity rotations can sustain soil biological communities, with positive effects on soil organic matter and soil fertility.

Mechanisms of soil carbon accrual and storage in bioenergy cropping systems

Annual row cropping systems converted to perennial bioenergy crops tend to accrue soil C, likely a function of increased root production and decreased frequency of tillage; however, very little is known about the mechanisms governing the accrual and stability of this additional soil C. To address this uncertainty, we assessed the formation and stability of aggregates and soil organic C (SOC) pools under switchgrass, giant miscanthus, a native perennial grass mix and continuous corn treatments in Michigan and Wisconsin soils differing in both texture and mineralogy. We isolated different aggregate size fractions, >2 mm, 0.5–2 mm, and <0.5 mm, using a procedure intended to minimize alterations to aggregate biological and chemical properties. We determined SOC, permanganate oxidizable C (POXC), and microbial activities (i.e. enzyme activities and soil respiration rates) associated with these aggregates. Soil type strongly influenced the trajectory of aggregate formation and stabilization with differences between sites in mean aggregate size, stability, SOC and microbial activity under perennial vs. corn cropping systems. At the Michigan site, soil microbial activities were highest in the >2 mm aggregates, and higher under the perennial grasses compared to corn. Contrastingly, in Wisconsin soils, microbial activities were highest in the <0.5 mm aggregates and evidence for soil C accrual under perennial grasses was observed only in a fast turnover pool in the <0.5 mm aggregate class. Our results help explain cross‐site variability in soil C accrual under perennial bioenergy crops by demonstrating how interactions between belowground productivity, soil type, aggregation processes and microbial communities influence the rates and extent of SOC stabilization. Bioenergy cropping systems have the potential to be low‐C energy sources but first we must understand the complex interactions controlling the formation and stabilization of SOC if we are to maximize soil C accrual.

CARBON CONTROLS ON NITROUS OXIDE PRODUCTION WITH CHANGES IN SUBSTRATE AVAILABILITY IN A NORTH AMERICAN GRASSLAND

Fluxes of nitrous oxide (N2O) are governed by the availability of substrate nitrogen (N), soil moisture, and soil organic carbon (SOC) concentration. Grassland management techniques such as fertilization and haying can influence both SOC and soil N transformations. In mesic grasslands, where SOC can be high compared with drier regions, it is unclear to what extent heterotrophic denitrifiers will respond to such management practices. Biomass removal via haying can reduce SOC, whereas fertilization decreases C:N ratios of plant residues, which can decrease or increase heterotrophic microbial activity, respectively. We experimentally manipulated haying and fertilization regimens on a grassland in northeastern Kansas and measured resulting inorganic N availability, field fluxes of N2O and potential denitrification, and litter C:N ratios to assess how these management practices may influence soil N2O evolution. We observed N2O fluxes that were an order of magnitude larger than those reported in drier grassland systems with lower SOC. The largest N2O fluxes observed were in fertilized and fertilized/hayed plots, immediately after precipitation events. Haying periodically mitigated N2O fluxes. Denitrification enzyme activity was lower in hayed plots than unhayed plots, and greater with glucose-C additions, indicating that the C substrate in these soils is an important driver of denitrification. The N2O fluxes that represented at least 0.1% of available inorganic N exhibited a weak negative relationship with C:N ratios of litter in fertilized/hayed plots, suggesting that the lower C:N ratios associated the shift to C3 plants with fertilization may promote soil N2O production, in addition to SOC and inorganic N availability.

Effects of soil nitrogen availability on rhizodeposition in plants: a review

BackgroundSoil contains the majority of terrestrial carbon (C), forming the foundation for soil fertility and nutrient cycling. One key source of soil C is root-derived C, or rhizodeposits, which signal and sustain microbes that cycle nutrients such as nitrogen (N). Although N availability can affect rhizodeposition both quantitatively and qualitatively, these effects remain poorly understood due to conflicting results among studies.ScopeHere, we review studies examining the influence of soil N availability on rhizodeposition. We conduct a quantitative analysis of the response of various rhizodeposition C pools to N availability, and assess methodological aspects potentially underlying the highly variable results among studies. We also review impacts of N availability on the composition and quality of rhizodeposits.ConclusionsEffects of N on rhizodeposition were strongly dependent upon the specific C pools considered and the units for reporting those pools. N additions increased nearly all rhizodeposit C pools when expressed on a per plant basis, and decreased rhizodeposition per unit fixed C for several C pools, while no rhizodeposition C pools were significantly altered when expressed per unit root mass. Nevertheless, N effects were generally mixed due to a combination of variation in experimental methods and species-specific responses. Overall, our review indicates several key challenges for better understanding the mechanistic links between N availability, plant physiology, and microbial function. Identifying such links would substantially improve our ability to predict C- and N-dynamics in changing ecosystems.

Minerals in the rhizosphere: overlooked mediators of soil nitrogen availability to plants and microbes

Despite decades of research progress, ecologists are still debating which pools and fluxes provide nitrogen (N) to plants and soil microbes across different ecosystems. Depolymerization of soil organic N is recognized as the rate-limiting step in the production of bioavailable N, and it is generally assumed that detrital N is the main source. However, in many mineral soils, detrital polymers constitute a minor fraction of total soil organic N. The majority of organic N is associated with clay-sized particles where physicochemical interactions may limit the accessibility of N-containing compounds. Although mineral-associated organic matter (MAOM) has historically been considered a critical, but relatively passive, reservoir of soil N, a growing body of research now points to the dynamic nature of mineral-organic associations and their potential for destabilization. Here we synthesize evidence from biogeoscience and soil ecology to demonstrate how MAOM is an important, yet overlooked, mediator of bioavailable N, especially in the rhizosphere. We highlight several biochemical strategies that enable plants and microbes to disrupt mineral-organic interactions and access MAOM. In particular, root-deposited low-molecular-weight exudates may enhance the mobilization and solubilization of MAOM, increasing its bioavailability. However, the competitive balance between the possible fates of N monomers—bound to mineral surfaces versus dissolved and available for assimilation—will depend on the specific interaction between mineral properties, soil solution, mineral-bound organic matter, and microbes. Building off our emerging understanding of MAOM as a source of bioavailable N, we propose a revision of the Schimel and Bennett (Ecology 85:591–602, 2004) model (which emphasizes N depolymerization), by incorporating MAOM as a potential proximal mediator of bioavailable N.