The effects of previous crop residue, sowing direction and slope length on phosphorus losses from eroded sediments under no-tillage
Introduction
Erosion of soil by water is the result of sediment detachment and runoff transport, and is accelerated by human activities, causing severe damage to agricultural land (Wischmeier and Smith, 1978; Nearing et al., 2017). Thus, water erosion is one of the most disturbing forms of soil degradation and has had a major impact on the earth's crust shaping the relief forms, considering long time scales (Foster, 1982; Poesen, 2018). In agricultural land, water erosion is responsible for the decrease or loss of soil productive capacity in some areas due to the reduction of soil depth during decades or centuries (Pimentel and Burgess, 2013; Nearing et al., 2017; Schick et al., 2017; Poesen, 2018). Also, erosion of agricultural areas is a significant contributor to water pollution, because nutrients and pesticides are lost via dissolution in runoff and trapped by eroded sediments, which may threaten water quality (Hernani et al., 1999; Mirás-Avalos et al., 2009; Rickson, 2014; Issaka and Ashraf, 2017; Zhang et al., 2017).
Among the most important nutrients translocated by erosion, phosphorus (P) appears to provide high environmental risk, because of its effects on aquatic ecosystems. Soil P is a poor water-soluble element, so that concentrations of dissolved forms are in the order of ppbs; however, it is strongly adhered to solid particles leading to much higher concentrations of P bound to sediments (Sharpley et al., 1995; Hernani et al., 1999; Barbosa et al., 2009). Thus, as the P levels in the soil increase, the potential for transferring P in a soluble (dissolved) and particulate (associated with eroding soil) forms to aquatic systems increases (Sharpley et al., 1994, 1995; McIsaac et al., 1995). Soluble P forms are more decisive than particulate P forms in activating eutrophication (Quinton et al., 2001; Sharpley, 2016). However, it should not be assumed that particulate P is inert in the aquatic system, because a fraction of that P is readily available and another fraction is a potential source of long-term available P for aquatic biota, which may contribute to eutrophication (Sharpley et al., 1995; Correl, 1998).
The concern of the scientific community about the transfer of P to surface water is becoming increasingly greater (Quinton et al., 2001; Sharpley, 2016). Remarkably, it has been estimated that 8.5 to 9.5 million tons of this element are transported to the oceans each year, which implies a rate of about eight times larger than the natural rate due to erosion by geological processes (Rockström et al., 2009). Losses of P causing eutrophication may be relatively low, less than 1 to 2 kg ha-1 year-1 (Hansen et al., 2002). Therefore, although the magnitude of soil P losses by erosion may be irrelevant from the agricultural point of view, it could be sufficiently high to deteriorate water quality (Alberts and Spomer, 1985; Sharpley et al., 1994; Hansen et al., 2002). If the high anthropogenic P supply flowing to the seas is maintained, it may be sufficient to cause serious consequences to the sustainability of marine ecosystems, with increased risks of global environmental change (Handoh and Lenton, 2003; Rickson, 2014). This strengthens the need to control P losses in agricultural areas, which are considered to be the largest diffuse source of this element for surface waters (Sharpley et al., 1994; Quinton et al., 2001; Qian et al., 2014; Sharpley 2016; Zhang et al., 2017).
No-tillage (NT) combined with soil cover by crop residues has been recognized as a sustainable soil management technique (Wischmeier and Smith, 1978; Cogo et al., 1996; Hernani et al., 1999; Merten et al., 2015; Bolliger et al., 2006; Brown et al., 2018). For example, in southern Brazil, it has been shown that NT reduced soil losses and water losses by 87 and 62%, respectively, compared to conventional tillage (CT) (Schick et al., 2017). Subsequently, NT also reduces losses of nutrients by erosion (Bertol et al., 2017).
In spite of the importance of soil cover (Foster, 1982), other factors may affect runoff, and soil and nutrient losses. For instance, soil cover combined with contour tillage has been found to increase efficiency of erosion control (Barbosa et al., 2009; Marioti et al., 2013) and to decrease P loads (Quinton et al., 2001; Barbosa et al., 2009; Zhang et al., 2017).
However, even under NT, soil cover protection by crop residues, may lose efficiency in soil erosion control, in the presence of residue failure (Foster et al., 1982a). During failure, the runoff shear stress overcomes the resistance of both residues and soil and as a result, soil is increasingly washed away. The point of failure is defined as the critical slope length for which soil cover is partly degraded and soil loss significantly increases (Foster et al., 1982a, 1982b). Surface soil degradation, due to rainfall energy or subsurface soil degradation caused by wheeling also may contribute to decreasing infiltration rate and increasing runoff even under NT (Li et al., 2009); therefore degradation of soil hydraulic properties could be a factor governing residue failure.
Compared with conventional tillage (CT), NT is much more efficient in reducing concentration and losses of particulate P forms than those of soluble P forms (Sharpley et al., 1994; Soileau et al., 1994; McIsaac et al., 1995). Also conservation tillage systems, particularly NT, reduce much more soil losses than water losses in relation to CT (Merten et al., 2015; Bispo et al., 2017). Therefore, the trade-offs between runoff flow rate and P concentration in the eroded sediments should be considered as part of the strategies to control soil P losses.
Results of previous field experiments in Brazil, showed residue failure and increasing soil losses, under NT, for critical lengths varying between 29 and 483 m (Bertol et al., 1997; Morais and Cogo, 2001). However, previous research did not consider the effects of residue failure on sedimentary P losses. This factor may not only substantially increase the rate of soil loss by erosion, once the critical slope length is exceeded, but it may also increase transfers of P from soil to water. Thus, residue failure under NT means a reduction in soil cover efficiency, and this may shorten the positive effects in controlling or slowing P losses to rates that are not harmful for water pollution.
Residues from three different previous crops applied at two different rates and sowing directions, both across the slope and up and down the slope were assessed in field plots of standard size, for various settings of slope length. The aim of this study was to explore the impact, of previous crop residue and sowing direction for various slope lengths, on phosphorus content and loads of eroded sediments under NT. The specific objectives were: 1) to evaluate the effects of maize, wheat and soybean residues applied at rates equivalent to 50% and 100% of the dry mass production, 2) to compare sowing cross to the slope and up and down slope and 3) to assess the influence of residue failure for the various treatments studied. This research is based on two hypotheses: i) increasing quantity of previous crop residues combined with contour sowing reduces P losses via eroded sediments, and ii) there is a need to account for residue failure when assessing slope length effects on increased P loads; this is because strategies for control of P losses may become less effective, if this factor is present.
Section snippets
Study site
Field experiments were carried out under simulated rainfall in the Southern Highlands of Santa Catarina state, Brazil during 2010 and 2011. The study site is located at 27º 43' S longitude, 50º 31' W latitude and an altitude of 800 m asl. Mean annual temperature is 15.7 °C, while on average, yearly rainfall is 1556 mm. The climate of this area is subtropical, mild temperate with few extremes (warm summers, but warmest month less than 22 °C), and humid (ample precipitation in all months, and
Concentration of phosphorus in sediments
During the three seasons with simulated rainfall experiments, concentrations of P in eroded sediments (Psed) ranged from 19 to 59 mg kg-1. Table 2 shows the values of Psed for initial, critical and final slope lengths in the treatments. Following a three way ANOVA test, on average, thus irrespective of slope length, Psed concentrations were not significantly different (P < 0.05) when comparing sowing direction and rate of crop residues. The highest Psed concentrations were obtained under
Conclusions
Total aboveground biomass production was higher for maize than for wheat and soybean; P concentration of eroded sediments was significantly higher using soybean than using maize residues as soil cover, while P losses were significantly higher by soybean residues compared to maize and wheat residues. With increasing slope length the P content in sediments decreased because removal of the surface layers of soil by erosion is becoming gradually large and the higher flow moves larger particles
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
We are grateful to CNPq (National Council for Scientific and Technological, Brazil), and CAPES (Coordination of Superior Level Staff Improvement, Brazil) for the financial support.
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