Cover crops have long been touted for their ability to reduce erosion, fix atmospheric nitrogen, reduce nitrogen leaching, and improve soil health. In recent decades, there has been resurgence in ...cover crop adoption that is synchronous with a heightened awareness of climate change. Climate change mitigation and adaptation may be additional, important ecosystem services provided by cover crops, but they lie outside of the traditional list of cover cropping benefits. Here, we review the potential for cover crops to mitigate climate change by tallying all of the positive and negative impacts of cover crops on the net global warming potential of agricultural fields. Then, we use lessons learned from two contrasting regions to evaluate how cover crops affect adaptive management for precipitation and temperature change. Three key outcomes from this synthesis are (1) Cover crop effects on greenhouse gas fluxes typically mitigate warming by ~100 to 150 g CO
2
e/m
2
/year, which is higher than mitigation from transitioning to no-till. The most important terms in the budget are soil carbon sequestration and reduced fertilizer use after legume cover crops. (2) The surface albedo change due to cover cropping, calculated for the first time here using case study sites in central Spain and Pennsylvania, USA, may mitigate 12 to 46 g CO
2
e/m
2
/year over a 100-year time horizon. And (3) Cover crop management can also enable climate change adaptation at these case study sites, especially through reduced vulnerability to erosion from extreme rain events, increased soil water management options during droughts or periods of soil saturation, and retention of nitrogen mineralized due to warming. Overall, we found very few tradeoffs between cover cropping and climate change mitigation and adaptation, suggesting that ecosystem services that are traditionally expected from cover cropping can be promoted synergistically with services related to climate change.
1. Ecological studies identifying a positive relationship between biodiversity and ecosystem services motivate projections that higher plant diversity will increase services from agroecosystems. ...While this idea is compelling, evidence of generalizable relationships between biodiversity and ecosystem services that could be broadly applied in agricultural systems is lacking. 2. Cover crops grown in rotation with cash crops are a realistic strategy to increase agroecosystem diversity. We evaluated the prediction that further increasing diversity with cover crop polycultures would enhance ecosystem services and multifunctionality in a 2-year study of eighteen cover crop treatments ranging in diversity from one to eight species. Five ecosystem services were measured in each cover crop system and regression analysis used to explore the relationship between multifunctionality and several diversity indices. 3. As expected, there was a positive relationship between species richness and multifunctionality, but it only explained a small fraction of variance in ecosystem services (marginal R² = 0-05). In contrast, indices of functional diversity, particularly the distribution of trait abundances, were stronger predictors of multifunctionality (marginal R² = 0-15-0-38). 4. Synthesis and application. In a corn production system, simply increasing cover crop species richness will have a small impact on agroecosystem services, but designing polycultures that maximize functional diversity may lead to agroecosystems with greater multifunctionality.
Cover crops (CCs) can increase soil organic carbon (SOC) sequestration by providing additional OC residues, recruiting beneficial soil microbiota, and improving soil aggregation and structure. The ...various CC species that belong to distinct plant functional types (PFTs) may differentially impact SOC formation and stabilization. Biogeochemical theory suggests that selection of PFTs with distinct litter quality (C:N ratio) should influence the pathways and magnitude of SOC sequestration. Yet, we lack knowledge on the effect of CCs from different PFTs on the quantity and composition of physiochemical pools of SOC. We sampled soils under monocultures of three CC PFTs (legume crimson clover; grass triticale; and brassica canola) and a mixture of these three species, from a long‐term CC experiment in Pennsylvania, USA. We measured C content in bulk soil and C content and composition in contrasting physical fractions: particulate organic matter, POM; and mineral‐associated organic matter, MAOM. The bulk SOC content was higher in all CC treatments compared to the fallow. Compared to the legume, monocultures of grass and brassica with lower litter quality (wider C:N) had higher proportion of plant‐derived C in POM, indicating selective preservation of complex structural plant compounds. In contrast, soils under legumes had greater accumulation of microbial‐derived C in MAOM. Our results for the first time, revealed that the mixture contributed to a higher concentration of plant‐derived compounds in POM relative to the legume, and a greater accumulation of microbial‐derived C in MAOM compared to monocultures of grass and brassica. Mixtures with all three PFTs can thus increase the short‐ and long‐term SOC persistence balancing the contrasting effects on the chemistries in POM and MAOM imposed by monoculture CC PFTs. Thus, despite different cumulative C inputs in CC treatments from different PFTs, the total SOC stocks did not vary between CC PFTs, rather PFTs impacted whether C accumulated in POM or MAOM fractions. This highlights that CCs of different PFTs may shift the dominant SOC formation pathways (POM vs. MAOM), subsequently impacting short‐ and long‐term SOC stabilization and stocks. Our work provides a strong applied field test of biogeochemical theory linking litter quality to pathways of C accrual in soil.
Plant functional types (PFTs) substantially impact the accrual and persistence of soil organic carbon (SOC) in particulate (POM) and mineral‐associated organic matter (MAOM). We investigated the effect of PFTs (e.g., legume, grass, brassica) on SOC in a long‐term crop cover (CC) experiment. CCs with low litter quality (grasses) accrued higher C in POM, abundant in plant‐derived compounds. However, CCs with high litter quality (legumes) contributed to higher microbial‐derived C in MAOM. Interestingly, mixtures with all PFTs contributed to both POM and MAOM, thus increasing the short‐and long‐term SOC persistence. These results suggest that plant trait‐based understanding of CC practices could improve SOC stocks.
Soil phosphorus (P) management remains a critical challenge for agriculture worldwide, and yet we are still unable to predict soil P dynamics as confidently as that of carbon (C) or nitrogen (N). ...This is due to both the complexity of inorganic P (Pi) and organic P (Po) cycling and the methodological constraints that have limited our ability to trace P dynamics in the soil–plant system. In this review, we describe the challenges for building parsimonious, accurate, and useful biogeochemical models that represent P dynamics and explore the potential of new techniques to usher P biogeochemistry research and modeling forward. We conclude that research efforts should focus on the following: (1) updating the McGill and Cole (1981) model of Po mineralization by clarifying the role and prevalence of biochemical and biological Po mineralization, which we suggest are not mutually exclusive and may co-occur along a continuum of Po substrate stoichiometry; (2) further understanding the dynamics of phytate, a six C compound that can regulate the poorly understood stoichiometry of soil P; (3) exploring the effects of C and Po saturation on P sorption and Po mineralization; and (4) resolving discrepancies between hypotheses about P cycling and the methods used to test these hypotheses.
We propose a method to improve the computational and memory efficiency of numerical solvers for the nonequilibrium Dyson equation in the Keldysh formalism. It is based on the empirical observation ...that the nonequilibrium Green's functions and self energies arising in many problems of physical interest, discretized as matrices, have low rank off-diagonal blocks, and can therefore be compressed using a hierarchical low rank data structure. We describe an efficient algorithm to build this compressed representation on the fly during the course of time stepping, and use the representation to reduce the
cost of computing history integrals, which is the main computational
bottleneck. For systems with the hierarchical low rank property, our
method reduces the computational complexity of solving the
nonequilibrium Dyson equation from cubic to near quadratic, and the
memory complexity from quadratic to near linear. We demonstrate the full
solver for the Falicov-Kimball model exposed to a rapid ramp and
Floquet driving of system parameters, and are able to
increase feasible propagation times substantially. We present examples with
262 144 time steps, which would require approximately five months
of computing time and 2.2 TB of memory using the direct time stepping method,
but can be completed in just over a day on a laptop with
less than 4 GB of memory using our method.
We also confirm the hierarchical low
rank property for the driven Hubbard model in the weak coupling regime
within the GW approximation, and in the strong coupling regime
within dynamical mean-field theory.
Cover crops have the potential to be agricultural nitrogen (N) regulators that reduce leaching through soils and then deliver N to subsequent cash crops. Yet, regulating N in this way has proven ...difficult because the few cover crop species that are well-studied excel at either reducing N leaching or increasing N supply to cash crops, but they fail to excel at both simultaneously. We hypothesized that mixed species cover crop stands might balance the N fixing and N scavenging capabilities of individual species. We tested six cover crop monocultures and four mixtures for their effects on N cycling in an organically managed maize-soybean-wheat feed grain rotation in Pennsylvania, USA. For three years, we used a suite of integrated approaches to quantify N dynamics, including extractable soil inorganic N, buried anion exchange resins, bucket lysimeters, and plant N uptake. All cover crop species, including legume monocultures, reduced N leaching compared to fallow plots. Cereal rye monocultures reduced N leaching to buried resins by 90% relative to fallow; notably, mixtures with just a low seeding rate of rye did almost as well. Austrian winter pea monocultures increased N uptake in maize silage by 40 kg N ha-1 relative to fallow, and conversely rye monocultures decreased N uptake into maize silage by 40 kg N ha-1 relative to fallow. Importantly, cover crop mixtures had larger impacts on leaching reduction than on maize N uptake, when compared to fallow plots. For example, a three-species mixture of pea, red clover, and rye had similar maize N uptake to fallow plots, but leaching rates were 80% lower in this mixture than fallow plots. Our results show clearly that cover crop species selection and mixture design can substantially mitigate tradeoffs between N retention and N supply to cash crops, providing a powerful tool for managing N in temperate cropping systems.
Cover crop mixtures can provide multiple ecosystem services but provisioning of these services is contingent upon the expression of component species in the mixture. From the same seed mixture, cover ...crop mixture expression varied greatly across farms and we hypothesized that this variation was correlated with soil inorganic nitrogen (N) concentrations and growing degree days. We measured fall and spring biomass of a standard five-species mixture of canola (Brassica napus L.), Austrian winter pea (Pisum sativum L), triticale (x Triticosecale Wittm.), red clover (Trifolium pratense L.) and crimson clover (Trifolium incarnatum L.) seeded at a research station and on 8 farms across Pennsylvania and New York in two consecutive years. At the research station, soil inorganic N (soil iN) availablity and cumulative fall growing degree days (GDD) were experimentally manipulated through fertilizer additions and planting date. Farmers seeded the standard mixture and a "farm-tuned" mixture of the same five species with component seeding rates adjusted to achieve farmer-desired services. We used Structural Equation Modeling to parse out the effects of soil iN and GDD on cover crop mixture expression. When soil iN and fall GDD were high, canola dominated the mixture, especially in the fall. Low soil iN favored legume species while a shorter growing season favored triticale. Changes in seeding rates influenced mixture composition in fall and spring but interacted with GDD to determine the final expression of the mixture. Our results show that when soil iN availability is high at the time of cover crop planting, highly competitive species can dominate mixtures which could potentially decrease services provided by other species, especially legumes. Early planting dates can exacerbate the dominance of aggressive species. Managers should choose cover crop species and seeding rates according to their soil iN and GDD to ensure the provision of desired services.
Nitrate can be reduced to other N inorganic species via denitrification and incorporated into organic matter by immobilization; however, the effect of biotic/abiotic and redox condition on ...immobilization and denitrification processes from a single system are not well documented. We hypothesize nitrate (NO3-) transformation pathways leading to the formation of dissolved- and solid-phase organic N are predominantly controlled by abiotic reactions, but the formation of soluble inorganic N species is controlled by redox condition. In this study, organic matter in the form of leaf compost (LC) was spiked with 15NO3- and incubated under oxic/anoxic and biotic/abiotic conditions at pH 6.5. We seek to understand how variations in environmental conditions impact NO3- transformation pathways through laboratory incubations. We find production of NH4+ is predominantly controlled by redox whereas NO3- conversion to dissolved organic nitrogen (DON) and immobilization in solid-phase N are predominantly controlled by abiotic processes. Twenty % of added 15N-NO3- was incorporated into DON under oxic conditions, with abiotic processes accounting for 85% of the overall incorporation. Nitrogen immobilization processes resulted in N concentrations of 4.1-6.6 μg N (g leaf compost)-1, with abiotic processes accounting for 100% and 66% of the overall (biotic+abiotic) N immobilization under anoxic and oxic conditions, respectively. 15N-NMR spectroscopy suggests 15NO3- was immobilized into amide/aminoquinones and nitro/oxime under anoxic conditions. A fraction of the NH4+ was produced abiotically under anoxic conditions (~10% of the total NH4+ production) although biotic organic N mineralization contributed to most of NH4+ production. Our results also indicate Fe(II) did not act as an electron source in biotic-oxic incubations; however, Fe(II) provided electrons for NO3- reduction in biotic-anoxic incubations although it was not the sole electron source. It is clear that, under the experimental conditions of this investigation, abiotic and redox processes play important roles in NO3- transformations. As climatic conditions change (e.g., frequency/intensity of rainfall), abiotic reactions that shift transformation pathways and N species concentrations from those controlled by biota might become more prevalent.
•Environment and cover crop mixture composition affect N supply/retention tradeoffs.•Low soil NO3−-N concentrations at cover crop planting minimize the tradeoff.•Mixtures with a low seeding rate of ...winter-hardy grasses minimize the tradeoff.•A graphical analysis tool visualizes and can inform management of the tradeoff.
The ability of cover crop mixtures to provide both nitrogen (N) retention and N supply services has been extensively studied in research station experiments, especially with grass-legume bicultures. Mixtures are often as effective as grass monocultures at N retention, but the N supply service can be compromised when non-legumes dilute the presence of legumes in a cover crop stand. To study the tradeoffs between N retention and supply when using cover crop mixtures, we measured N retention and supply in distributed on-farm experiments, developed multiple linear regression models to predict N retention and supply based on cover crop functional characteristics and environmental variables, and synthesized the regression models into a graphical analysis tool. The experiments took place on three organic farms and a research station in Pennsylvania, USA and tested 3-species and 4-species cover crop mixtures in comparison to commonly used grass and legume monocultures. Cover crop treatments were planted between a small grain crop harvested in mid-summer and a maize (Zea mays L.) crop planted the following spring. Potential nitrate (NO3−) leaching below 30cm, an indicator of the N retention service, declined as the presence of non-legume species in a cover crop increased (r2=0.72). Potential NO3− leaching increased as the August soil NO3−-N concentration increased and as the fall biomass N content of winter-killed species or canola (Brassica napus L. ‘Wichita’) increased. Relative maize yield, an indicator of the N supply service, decreased as fall and spring cover crop biomass carbon-to-nitrogen (C:N) ratios increased and increased as total spring biomass N content and soil carbon (C) concentration increased (r2=0.56). Synthesizing the regression models in a graphical analysis tool revealed a tradeoff between N supply and retention services for cover crop mixtures, where increasing the fractional non-legume seeding rate to reduce potential NO3− leaching also reduced relative maize yield. The tradeoff could be minimized by managing environmental conditions and cover crop composition so that potential NO3− leaching remains low even when the fractional non-legume seeding rate is low. The regression models suggest this could be achieved by maintaining low soil NO3−-N concentrations prior to cover crop planting in August, not including winter-killed legumes in the mixture, and using non-legume species that are the most efficient at N retention. Thus, with thoughtful management of cover crops and soils, farmers may be able to realize the potential of cover crop mixtures to provide high levels of both N retention and supply services.
Soil fertility in organic agriculture relies on microbial cycling of nutrient inputs from legume cover crops and animal manure. However, large quantities of labile carbon (C) and nitrogen (N) in ...these amendments may promote the production and emission of nitrous oxide (N₂O) from soils. Better ecological understanding of the N₂O emission controls may lead to new management strategies to reduce these emissions. We measured soil N₂O emission for two growing seasons in four corn–soybean–winter grain rotations with tillage, cover crop, and manure management variations typical of organic agriculture in temperate and humid North America. To identify N₂O production pathways and mitigation opportunities, we supplemented N₂O flux measurements with determinations of N₂O isotopomer composition and microbiological genomic DNA abundances in microplots where we manipulated cover crop and manure additions. The N input from legume-rich cover crops and manure prior to corn planting made the corn phase the main source of N₂O emissions, averaging 9.8 kg/ha of N₂O-N and representing 80% of the 3-yr rotations’ total emissions. Nitrous oxide emissions increased sharply when legume cover crop and manure inputs exceeded 1.8 and 4 Mg/ha (dry matter), respectively. Removing the legume aboveground biomass before corn planting to prevent co-location of fresh biomass and manure decreased N₂O emissions by 60% during the corn phase. The co-occurrence of peak N₂O emission and high carbon dioxide emission suggests that oxygen (O₂) consumption likely caused hypoxia and bacterial denitrification. This interpretation is supported by the N₂O site preference values trending towards denitrification during peak emissions with limited N₂O reduction, as revealed by the N₂O δ15N and δ18O and the decrease in clade I nosZ gene abundance following incorporation of cover crops and manure. Thus, accelerated microbial O₂ consumption seems to be a critical control of N₂O emissions in systems with large additions of decomposable C and N substrates. Because many agricultural systems rely on combined fertility inputs from legumes and manures, our research suggests that controlling the rate and timing of organic input additions, as well as preventing the co-location of legume cover crops and manure, could mitigate N₂O emissions.
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