Evaluating cover crops as a climate change adaptation strategy

Common agronomic adaptation strategies to climate change may increase soil greenhouse gas emission in Northern Europe

In the Chesapeake Bay Watershed, winter cereal cover crops are often planted in rotation with summer crops to reduce the loss of nutrients and sediment from agricultural systems. Cover crops can also improve soil health, control weeds and pests, supplement forage needs, and support resilient cropping systems. In southeastern Pennsylvania, cover crops can be successfully established following corn (Zea mays L.) silage harvest and are strongly pro- moted for use in this niche. They are also planted following corn grain, soybean (Glycine max L.), and vegetable harvest. In Pennsylvania, the use of winter cover crops for agricultural con- servation has been supported through a combination of outreach, regulation, and incentives. On-farm implementation is thought to be increasing, but the actual extent of cover crops is not well quantified. Satellite imagery can be used to map green winter cover crop vegetation on agricultural fields and, when integrated with additional remote sensing data products, can be used to evaluate wintertime vegetative groundcover following specific summer crops. This study used Landsat and SPOT (System Probatoire d' Observation de la Terre) satellite imagery, in combination with the USDA National Agricultural Statistics Service Cropland Data Layer, to evaluate the extent and amount of green wintertime vegetation on agricultural fields in four Pennsylvania counties (Berks, Lebanon, Lancaster, and York) from 2010 to 2013. In December of 2010, a windshield survey was conducted to collect baseline data on winter cover crop implementation, with particular focus on identifying corn harvested for silage (expected earlier harvest date and lower levels of crop residue), versus for grain (expected later harvest date and higher levels of crop residue). Satellite spectral indices were successfully used to detect both the amount of green vegetative groundcover and the amount of crop residue on the surveyed fields. Analysis of wintertime satellite imagery showed consistent increases in vegetative groundcover over the four-year study period and determined that trends did not result from annual weather variability, indicating that farmers are increasing adoption of practices such as cover cropping that promote wintertime vegetation. Between 2010 and 2013, the occurrence of wintertime vegetation on agricultural fields increased from 36% to 67% of corn fields in Berks County, from 53% to 75% in Lancaster County, from 42% to 65% in Lebanon County, and from 26% to 52% in York County. Apparently, efforts to promote cover crop use in the Chesapeake Bay Watershed have coincided with a rapid increase in the occurrence of wintertime vegetation following corn harvest in southeastern Pennsylvania. However, despite these increases, between 25% and 48% of corn fields remained without substantial green vegetation over the wintertime, indicating further opportunity for cover crop adoption.

Effects of aged biochar additions at different addition ratios on soil greenhouse gas emissions

As the name implies, a cover crop consists of plants grown primarily to keep the land covered, especially during the off-season or between cash crops. In temperate regions like most of Europe and North America, a cover crop sown immediately after the main crop harvest in fall is considered a winter cover crop. It will grow in the fall, either subjected to frostkill or go into relative dormancy during the dead of winter, and then, if winter-hardy, recommence growth in very early spring before soils are warm and dry enough for the next cash crop. If the climate is sufficiently mild, such cover crops may produce substantial above-ground dry matter (3000–6000 kg ha−1) and nearly complete ground cover before being terminated. For many decades, the use of cover crops has been promoted mainly to prevent the severe soil erosion that winter and spring rains can bring if soils are left bare. In addition, it is widely recognized that regular use of winter cover crops – as compared to bare fallow over winter – can provide enough carbon input to build – or at least slow the decline of – soil organic matter. For these reasons, many scientists view cover crops as an essential tool in managing farmland for long-term sustainability.1 A considerable amount of cover crop research has been conducted in the mid-Atlantic region of the USA during the past three decades. Most of this research focused on just a handful of cover crop species, mainly cereal rye and hairy vetch, which were found to be well adapted to the region’s climate and cropping systems. With the advent of programs to restore the health of the Chesapeake Bay, most cover crop research in Maryland has been directed towards using cover crops to capture residual mineral nitrogen (N) before it can leach away in the fall. Extensive research on coastal plain soils has demonstrated the ability of a rye cover crop to greatly reduce the loss of N to groundwater from maize grown in no-till production systems.2 However, relatively little has been done to demonstrate direct benefits to the farmer from the use of cover crops. One economic benefit that has been well quantified is the ability of legume cover crops, under some conditions, to replace by biological N2 fixation most or all of the fertilizer N needed for optimal production of nitrogen-demanding crops. However, under realistic conditions, research indicates that it costs about as much to grow and manage a hairy vetch cover crop as the value of the N fertilizer it saves.3,4 While biologically fixed N is likely to become more profitable as the cost of N fertilizer rises, legume cover crops grown alone are not very effective at capturing residual fall N. Although not often discussed by researchers, farmers recognize that growing a cover crop adds extra expense, complexity and uncertainty to the already risky business of farming. Under some circumstances, certain cover crops have interfered with crop production by using up water stored in the soil profile, by immobilizing N needed for the cash crop and by becoming weedy or producing excessive residues, hampering crop stand establishment or harvest. The most obvious direct costs associated with cover crops include those for cover crop seed, labor, fuel, fertilizer and herbicide or tillage to kill the cover crop. Given these considerations, the State of Maryland has for several years paid subsidies of $50–100 per hectare for timely planting of cover crops with a goal of keeping at least 75% of Maryland’s cropland acres under cover crops in winter. Despite this incentive, adoption rates remain relatively low, with only about 20–25% of cropland hectares receiving cover crops. We suspect that this is because most farmers in this region are not sufficiently aware of the direct benefits that cover crops potentially offer, possibly as a result of past research and extension work that emphasized cover cropping’s role in N fixation, environmental protection and long-term soil resource conservation. Although nearly all farmers desire to be good stewards of their land, most face tight (or negative) profit margins and cannot afford to engage in environmental altruism without first considering their operations’ bottom line and efficiency goals. Lacking credible information and examples that might convince them otherwise, many farmers have reached the conclusion that cover crops are simply not worth the cost and trouble.

Effects of different agricultural organic wastes on soil GHG emissions: During a 4-year field measurement in the North China Plain

Using NASA Earth observations and Google Earth Engine to map winter cover crop conservation performance in the Chesapeake Bay watershed

Mitigating greenhouse gas emissions and ammonia volatilization from cotton fields by integrating cover crops with reduced use of nitrogen fertilizer

Using cost-benefit analysis to understand adoption of winter cover cropping in California's specialty crop systems

Diversifying agronomic production systems by combining crops and livestock (i.e., Integrated Crop Livestock systems; ICL) may help mitigate the environmental impacts of intensive single-commodity production. In addition, harvesting row-crop residues and/or perennial biomass could increase the multi-functionality of ICL systems as a potential source for second-generation bioenergy feedstock. Here, we evaluated non-CO2 soil greenhouse gas (GHG) emissions from both row-crop and perennial grass phases of a field-scale model ICL system established on marginally productive, poorly drained cropland in the western US Corn Belt. Soil emissions of nitrous oxide (N2O) and methane (CH4) were measured during the 2017–2019 growing seasons under continuous corn (Zea mays L.) and perennial grass treatments consisting of a common pasture species, ‘Newell’ smooth bromegrass (Bromus inermis L.), and two cultivars of switchgrass (Panicum virgatum L.), ‘Liberty’ and ‘Shawnee.’ In the continuous corn system, we evaluated the impact of stover removal by mechanical baling vs. livestock grazing for systems with and without winter cover crop, triticale (x Triticosecale neoblaringhemii A. Camus; hexaploid AABBRR). In perennial grasslands, we evaluated the effect of livestock grazing vs. no grazing. We found that (1) soil N2O emissions are generally higher in continuous corn systems than perennial grasslands due to synthetic N fertilizer use; (2) winter cover crop use had no effect on total soil GHG emissions regardless of stover management treatment; (3) stover baling decreased total soil GHG emissions, though grazing stover significantly increased emissions in one year; (4) grazing perennial grasslands tended to increase GHG emissions in pastures selected for forage quality, but were highly variable from year to year; (5) ICL systems that incorporate perennial grasses will provide the most effective GHG mitigation outcomes.

Characteristics of greenhouse gas emissions from farmland soils based on a structural equation model: Regulation mechanism of biochar

Many agricultural regions in China are likely to become appreciably wetter or drier as the global climate warming increases. However, the impact of these climate change patterns on the intensity of soil greenhouse gas (GHG) emissions (GHGI, GHG emissions per unit of crop yield) has not yet been rigorously assessed. By integrating an improved agricultural ecosystem model and a meta‐analysis of multiple field studies, we found that climate change is expected to cause a 20.0% crop yield loss, while stimulating soil GHG emissions by 12.2% between 2061 and 2090 in China's agricultural regions. A wetter‐warmer (WW) climate would adversely impact crop yield on an equal basis and lead to a 1.8‐fold‐ increase in GHG emissions relative to those in a drier‐warmer (DW) climate. Without water limitation/excess, extreme heat (an increase of more than 1.5°C in average temperature) during the growing season would amplify 15.7% more yield while simultaneously elevating GHG emissions by 42.5% compared to an increase of below 1.5°C. However, when coupled with extreme drought, it would aggravate crop yield loss by 61.8% without reducing the corresponding GHG emissions. Furthermore, the emission intensity in an extreme WW climate would increase by 22.6% compared to an extreme DW climate. Under this intense WW climate, the use of nitrogen fertilizer would lead to a 37.9% increase in soil GHG emissions without necessarily gaining a corresponding yield advantage compared to a DW climate. These findings suggest that the threat of a wetter‐warmer world to efforts to reduce GHG emissions intensity may be as great as or even greater than that of a drier‐warmer world.

Agricultural nutrient runoff to the Chesapeake Bay has been under intense scrutiny for more than a decade in Maryland. One method for capturing these nutrients, especially N, is the use of winter cover crops. This study compared various broadcast cover crop treatments with and without soil incorporation to planting winter cover crop seed with a no‐till drill. Seedling emergence and N uptake were the dependent variables measured for two planting dates and seven planting methods. The effects of planting date and planting method for winter wheat (Triticum aestivum L.) and cereal rye (Secale cereale L.) following corn (Zea mays L.) harvest were investigated at two locations. The study was conducted over two winter cover crop growing seasons: 2007–2008 and 2008–2009. Treatments that incorporated the seed into the soil consistently established better stands of cover crops and took up more N regardless of fluctuations in temperature, rainfall, and planting date. Early planted cover crops consistently took up more N than those planted on the later planting date. Performance of the broadcast treatments was highly dependent on rainfall and mild temperatures for success, but did take up notable amounts of N when planted early under good growing conditions. The few differences that were found in the N uptake between wheat and rye within the same planting treatment always indicated that the rye achieved better N uptake than wheat.

Vegetable production primarily relies on the conventional tillage system (CTS), which leads to soil degradation through erosion and reduced soil health. The use of no-tillage vegetable systems (NTVS) aims to mitigate these issues; however, information about the impact of this management system on soil health and greenhouse gas (GHG) emissions remains limited. Thus, the objective of this study was to conduct an on-farm evaluation of the effects of no-tillage and cover crop use on soil C and N contents and stocks, soil bulk density (SD), mean geometric diameter (MGD) of aggregates, soil temperature, volumetric soil moisture (VM), plant yield, and GHG emissions in cauliflower production under NTVS compared to CTS in a subtropical ecosystem in southeastern Brazil. Chemical and physical properties were assessed at depths of 0–5, 5–10, and 10–30 cm. GHG emissions, particularly nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) were measured using closed static chambers and gas chromatography. NTVS with cover crop mixes had higher yield than CTS without cover crops (25.1 and 18.4 Mg ha−1, respectively). NTVS exhibited increased MGD and VM and reduced SD. Soil temperature in the 0–5 cm layer was lower in NTVS than in CTS. Soil C and N stocks were higher in NTVS, but high N2O emissions offset this advantage compared to CTS. Overall, NTVS emitted more CO2 and N2O than CTS, while both systems showed soil CH4 uptake. NTVS maintained sufficient carbon equivalent reserves (0–30 cm) to offset GHG emissions, making it a viable alternative for plant yield and soil quality; however, its environmental impact on GHG emissions requires further attention.

Potentials to mitigate greenhouse gas emissions from Swiss agriculture

In-field measurements of direct soil greenhouse gas (GHG) emissions provide critical data for quantifying the net energy efficiency and economic feasibility of crop residue-based bioenergy production systems. A major challenge to such assessments has been the paucity of field studies addressing the effects of crop residue removal and associated best practices for soil management (i.e., conservation tillage) on soil emissions of carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). This regional survey summarizes soil GHG emissions from nine maize production systems evaluating different levels of corn stover removal under conventional or conservation tillage management across the US Corn Belt. Cumulative growing season soil emissions of CO2, N2O, and/or CH4 were measured for 2–5 years (2008–2012) at these various sites using a standardized static vented chamber technique as part of the USDA-ARS’s Resilient Economic Agricultural Practices (REAP) regional partnership. Cumulative soil GHG emissions during the growing season varied widely across sites, by management, and by year. Overall, corn stover removal decreased soil total CO2 and N2O emissions by -4 and -7 %, respectively, relative to no removal. No management treatments affected soil CH4 fluxes. When aggregated to total GHG emissions (Mg CO2 eq ha−1) across all sites and years, corn stover removal decreased growing season soil emissions by −5 ± 1 % (mean ± se) and ranged from -36 % to 54 % (n = 50). Lower GHG emissions in stover removal treatments were attributed to decreased C and N inputs into soils, as well as possible microclimatic differences associated with changes in soil cover. High levels of spatial and temporal variabilities in direct GHG emissions highlighted the importance of site-specific management and environmental conditions on the dynamics of GHG emissions from agricultural soils.