Cover Crops Effects on Soil Erosion and Water Quality
Introduction
The term ‘cover crops’ is used to describe the crops as one of the components of the sustainable agricultural management practices, that cover the soil to protect from erosion, improve soil quality and enhance agio-ecosystem services. These crops are used during fallow periods in annual grain systems. Both grasses and legumes are widely used as cover crops. Legumes are usually used as cover crops for green manuring, as they fix the nitrogen (N) from the atmosphere via biological N fixation in soil to provide it to the grain crops. In winter, small grain crops, such as winter lye, wheat, barley, oats and triticale are used as cover crops. These crops are generally managed by mechanical methods or herbicide application before the planting of main seasonal crops, like com, soybeans, sunflower, etc. Among the legumes, some excellent cover crops include alfalfa, cowpeas, winter peas. Sunn hemp and red. white and sweet clover. Glasses and Brassicas, such as ryegrass, mustard and radish can also potentially be used as cover crops. The benefits of cover crops include N fertility and organic matter, protection from soil erosion, increased soil moisture, capture and recycling of nutrients, decreased soil compaction and suppression of weeds, nematodes and other soil-bome diseases (Fig. 1).
Fig. 1 Cover crops reduce soil erosion (Source: Uttech, 2010). Cover crops reduce erosion, runoff (public release: 18 May
2010. http://www.agronomy.org).
In a natural ecosystem, the soil is covered most of the time by growing plants that maintain soil moisture through the transpiration of water, increasing rainfall infiltration, fixing carbon, keepmg reactive nutrients like N and phosphorus (P) in place, preventing runoff and supporting soil flora and fauna. In an annual cropping system, management practices affect residues distribution and placement, leaving the soil partially exposed and unprotected to the erosive forces of wind and water, loss of nutrient and organic matter, increased runoff and stressed soil organisms with low soil productivity during the fall, winter, and early spring. Thus, planting cover crops during fallow periods helps to maintain soil health and protect water quality.
Duran and Pleguezuelo (2008) have studied the effects of cover crops on soil and observed that an increased duration of plant cover is an effective strategy to reduce N and P losses from annual crops, like com silage, but the degree of improvement depends on management. Cereal lye (Secale cereal L.) planting after com silage harvest provides an over-the-winter soil cover, but post-harvest planting does not address the risk of soil and nutrient losses that occur during silage production (Wendt and Bunvell, 1985). To prevent nutrient losses during and after com silage production, Siller et al. (2016) used living mulch systems with Kura clover (Trifolium ambiguum L.) alone and in combination with a cereal lye cover crop.
Inter-seeding of cover crops with com have reportedly reduced nutrient losses following the harvest or during com production (Kieinman et al., 2005; Grabber and Jokela, 2013; Grabber, 2014). It is reported that cover crops are only helpfi.il when managed properly, otherwise they can lead to yield decline of mam crops, though contributing little to farm profitability due to market constraints (Sarrantonio and Gallandt, 2003; Roesch-McNally et al., 2017). However, current technologies and selection of economically valuable and suitable cover crops can overcome these barriers and improve agro-ecosystem services. Inter-seeding of alfalfa with com silage within a few days of com planting provides a valuable forage crop, as well as a good cover crop as seen in a research experiment conducted in Wisconsin (Osterholz et al., 2019). Com silage-alfalfa rotations achieve double the alfalfa production than that through solo seeding of alfalfa in the first year of its application (Grabber, 2016; Osterholz et al., 2017).
Effectiveness of Cover Crop in Protecting Ecosystem Resources
In recent decades, agricultural production has been intensified to meet global food security and this is inconsistent with environmental impact and ecological resources (soil, water, or air quality) deterioration (Tilman, 2002; Lai, 2008). Mono-cropping, over-fertilisation, pesticide application, overgrazing, nutrient leaching, fallow periods without cover crops and excessive tillage are some of the conventional management practices that convert once-fertile agricultural lands mto degraded and marginal lands (Scherr and Yadav, 1996; Maharjan et al., 2016). Agricultural land degradation is one of the great concerns to farm income, ecosystem resources and decreased yield as has been most frequently observed (and still increasing) in Asia (206 M ha) (Oldemau, 1994; Scherr and Yadav, 1996). The United Nations Convention to Combat Desertification has postulated global zero net land degradation by 2030 (Stavi and Lai, 2015; UNCCD, 2012).
Soil erosion and water quality not only depend on agricultural management practices, but also natural factors, such as namral vegetation, climate and topography. Generally, for agricuhural lands, soil-loss rates are tenfold higher than soil-formation rates (Pimentel et al., 1987). Bare soils on slopes are at higher risk of erosion due to the intensity of rainfall and surface runoff (Morgan, 2005). Over-fertilisation also contributes to higher nutrient runoff through surface flow and percolation into aquatic ecosystems, thus deteriorating the freshwater resources. A study by Tilman et al. (2002) reported that only 30-50 per cent of N and 45 per cent of phosphorus is taken in by crops and the rest of the nutrients are lost from the fields.
To reduce pollution load, improve water quality and prevent soil erosion, best management practices (BMPs) for sustainable agriculture have been adapted worldwide. In addition to policies, incentives for fanners to implement BMPs, awareness and education about suitable cover crops in the area to maintain productive farmlands need to be provided. Sustainable agriculture ensures an economically-viable yield, environmental benefits and watershed and water quality protection, hi a smdy, Mahaijan et al. (2016) recommended that split fertiliser application and cover crop integration in cropping systems reduce the sediment load and nitrate and soluble reactive P levels significantly when compared to conventional agricultural management practices.
