Soil Health Benefits

Cover crops impact soil health by improving soil biodiversity and efficiency, chemical balancing, sequestering carbon, maintaining C:N stoichiometry and soil physical stability with moisture conservation and soil-erosion control (Islam et al., 2020). A significant increase in the size of the soil microbial biomass and higher metabolic efficiency under cover crops was due to the added effects of riiizo deposition from fine roots production (Bradford et al., 2013; Cotmfo et al., 2013). Cover crop roots exude hrto the rhizospliere a wide range of diverse labile C and N compounds (sugars, amino acids and organic acids), which are preferentially utilised by microbes and help to promote biological diversity and efficiency as natural diversity closely related to natural growth and efficiency (Cauarini et al., 2019).

Cover crops add diverse organic matter to soil that is usually returned to the soil after termination and subsequently, replenish SOM. Several studies have reported an increase in the TOC and TN contents under cover crops (Hessen et al., 2004; Plaza-Bonilla et al., 2016); while others have reported no change or a non-significant increase in TOC and TN contents (Jokela et al., 2009; Frasier et al., 2016). A significantly higher active organic C (a measure of soil quality) and active N content suggests an efficient biochemical response to the cover crop-induced changes that contribute to C :N stoichiometry in SOM.

Bulk density, as a measure of soil compaction, was significantly reduced by cover crop managements (Anonymous, 2019). Brassicas, such as radish, provide large quantities of residue on the surface soil and improve soil porosity and water infiltration properties. Cover crops stabilise soil structures by the physical and biochemical actions of fine roots, particularly when grass species, such as cereal lye, annul ryegrass, wheat, spelt and triticale are grown in the fall. Islam et al. (2020) reported that soil aggregate stability increased by 15 per cent under cover cropping compared to control.

Effect of radish on soil compaction (photo by Randall Reeder)

Fig. 5 Effect of radish on soil compaction (photo by Randall Reeder).

A good blend of cover crops increases and conserves soil moisture by allowing more water (rainfall, snowmelt, or irrigation) to infiltrate the soil than the bare fields. Cover-crop roots create soil macropores that hold moisture, keep the soil cooler, decrease irrigation frequency and reduce susceptibility of crops to abiotic stress, such as drought in rain-fed and dry areas. Moreover, the organic mulching effects of cover crops upon termination help to reduce evaporation demand. Karlen et al. (1994) reported that surface residue accumulation plays an important role in improving soil and biological, chemical and physical properties in no-tillage systems and consequently, reduce erosion and improve soil quality (Hargrove, 1991; Karlen et al. 1994).

Water Quality

Generally, compacted soil affects water infiltration with an associated increase in surface runoff and erosion, which are serious problems to long-term soil productivity (Pimentel et al., 1995). Soil erosion from bare fields carries sediment-laden soluble reactive P (SRP) and other nutrients into surface waters, and thus are responsible for algal blooms and degrading fisheries, sports and aquatic habitats (Hoomian and Sundemieier, 2017). Moreover, soil compaction affects plant root growth, water and nutrient availability to plants, and consequently, decreases crop yields.

Cover crops provide support to hold the soil and residue in the field (Fig. 6). They act as a living or dead biological mulch to control soil erosion by increasing water infiltration and permeability, improving groundwater recharge and storage and reducing offsite movement of SRP and other nutrients to freshwater systems to minimise algal blooms. A field study, conducted to evaluate the effect of cover crops on soil erosion and residue movement, showed that living cover crops (cereal rye) act as barriers to slow down soil and residue from conventionally-tilled bare fields. The aboveground biomass of cover crops act as cushions to reduce the impact of rainfall and slows down water movement, while its roots increase soil aggregate stability and form macropores through which water infiltrates, thus reducing surface nmoff. Cover crops also reduce nitrate leaching between 40-70 per cent compared with bare soil (Hoomian and Sundemieier, 2017) and reduce the chances of groundwater and drinking water contamination.

Cereal iye, as a winter cover crop, acts as a barrier to slows down soil and residue loss from the adjacent conventionally-

Fig. 6 Cereal iye, as a winter cover crop, acts as a barrier to slows down soil and residue loss from the adjacent conventionally-

tilled field.

Ecological and Environmental Benefits

Cover crops provide several ecological and environmental benefits, such as increased biodiversity and efficiency by attracting beneficial insects, reduction in greenhouse gas (GHG) emissions, enhanced aesthetic values and improved ecosystem services.

One of the major challenges of sustainable agricultural-management practices is to mitigate agriculture’s contribution to GHG emissions, especially N,0 emissions, into the atmosphere (Hu et al., 2015). Globally, agricultural soil ecosystems constitute the largest source ofN-,0 emissions (estimated at 6.8 Tg N,0-N year'1), comprising approximately 65 per cent of the total N,0 emitted into the atmosphere, with 4.2 Tg N20-N year'1 derived from N fertilisation and indirect emissions,

2.1 Tg N20-N year'1 arising from manure management and 0.5 Tg N20-N year"1 introduced through biomass burning (IPCC, 2007).

Several species of ammonia oxidiser, uitrifier denitrifiers and heterotrophic denitrifiers bacteria convert excess soil N into the N20, which traps 300 times more heat in the atmosphere than C02 (Zhu et al., 2013; Shcherbak et al., 2014; Hu et al., 2015). Cover crops, by up-taking N tied up in biomass and SOM, control the pathways of N20 emissions in the soil. In some cases, cover crops can even increase carbon storage, thereby regulating C:N stoichiometry in SOM, further reducing N20 emissions in the atmosphere (Mitchell et al., 2013).

The surface deposition and accumulation of winter-killed or herbicide-terminated cover-crop biomass often encourage the growth of diverse predatory insects that help control pests and diseases, thus minimising both herbicide and insecticide applications. Several cover crops can also kill harmful and parasitic disease-causing soil organisms, thus reducing the application of fungicides or nematicides (Snapp et al., 2007; Wen et al., 2017).

Herbicide-resistant weed pressures are increasing day by day. Weed pressure is expected to increase competition with agronomic crops as the global climate change effects continue to accelerate. While cover crops may not eliminate the need for herbicide application, they can suppress the weeds with competition for water, nutrients, space and light and thus, reduce herbicide applications. In some cases, green plantings have been shown to reduce weed density by 90 per cent in com crops (Teasdale, 1998; Anonymous, 2019).

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