Bibliografiska uppgifter för
Chemical treatment of cover crops - effects on leaching of N, P and glyphosate
Författare Utgivningsår Aronsson H., Ulén B., Stenberg M. 2008
Tidskrift/serie NJF Report Nr/avsnitt Ingår i... Utgivare Redaktör Huvudspråk Målgrupp Nummer (ISBN, ISSN) 4 Phosphorous management in Nordic-Baltic agriculture - reconciling productivity and environmental protection Nordiska jordbruksforskares förening (NJF) Rubæk G.H. Engelska Forskare ISSN 1653-2015
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NJF Report • Vol 4 • Nr 4 • Year 2008
Chemical treatment of cover crops – effects on leaching of N, P and glyphosate
Helena Aronsson, Barbro Ulén, and Maria Stenberg Department of Soil and Environment, Swedish University of Agricultural Sciences, PO Box 7014, SE-750 07 Uppsala Helena.aronsson@mark.slu.se
Introduction Cropping systems with crop cover during autumn and winter have a low risk of plant nutrient leaching. In crop rotations with cereals, undersown cover crops efficiently reduce nitrogen (N) leaching and may also reduce losses of phosphorus (P). Cover crops are often treated with glyphosate before incorporation into the soil. This study investigated how to minimise the risk of various negative environmental effects in cropping systems with cover crops. The effects of time of tillage after chemical treatment on leaching of N, P and glyphosate were studied in a sand and a clay soil in Sweden. The study was funded by the Swedish Council for Agricultural Research. Experimental design During 2005-2007, studies were carried out in south-west Sweden in field plots with separate tile-drainage systems, with three replicates on a sandy soil (Lilla Böslid, Halland, 70-90% sand) and two replicates on a clay soil (Lanna, Västergötland, 45% clay). Perennial ryegrass (Lolium perenne L.) cover crops were undersown in spring cereals. Different times for chemical treatment were tested in combination with soil tillage in autumn or in the following spring (Table 1). Drainage water was sampled continuously in proportion to water flow and analysed for N, P and glyphosate, while crops and soils were sampled occasionally. Crop yields of a following cereal crop were determined in order to study the residual effects of different treatments. Results Tillage strategies and effects on crop yields Different tillage strategies were used in the two experiments. On the sandy soil at Lilla Böslid, chemical wilting of the cover crops at different times was followed by
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mouldboard ploughing in November or in spring, except for one treatment where the cover crop was grown until spring without use of glyphosate. In this treatment, there was some growth of couch-grass (Elymus repens), which slightly affected the yields of the following cereal crop. On the clay soil at Lanna, chemical treatment in spring (with and without a cover crop) was followed by shallow cultivation (6 cm depth) before seedbed preparation. This resulted in problems with crop residues and a rough seedbed, especially in treatment C (Table 1) where it contributed to reduced crop yields. Table 1. Experimental treatments at the two field sites, measured drainage and leaching of N and P. Glyphosate (Glyphomax Bio, 3.5 -4 L/ha) was used for chemical treatment in autumn or spring. At Lilla Böslid soil tillage comprised stubble cultivation followed by ploughing. At Lanna the soil was ploughed in autumn or shallow-cultivated in spring (6 cm depth). Cover crop Lilla Böslid, sandy A Per. ryegrass B Per. ryegrass C Per. ryegrass D Per. ryegrass E Per. ryegrass F Per. ryegrass Lilla Böslid, sandy A Per. ryegrass B Per. ryegrass C Per. ryegrass D Per. ryegrass E Per. ryegrass F Per. ryegrass Chemical Soil tillage treatment soil, 2005/2006 26 Sep 24 Nov 26 Sep 12 Apr 4 Oct 24 Nov 4 Oct 12 Apr 31 Oct 12 Apr no 12 Apr soil, 2006/2007 26 Sep 24 Nov 26 Sep 2 Apr 10 Oct 24 Nov 10 Oct 2 Apr 22 Nov 2 Apr no 2 Apr 11 28 28 11 28 10 12 12 10 12 Nov Apr Apr Nov Apr Nov Apr Apr Nov Apr Drainage Leaching (kg/ha) (mm) PO4-P 432 450 436 336 439 487 614 676 655 691 654 649 92 149 189 202 206 290 295 357 407 392 25 25 22 17 20 22 41 39 37 41 27 22 2.0 4.4 1.7 7.4 8.8 10.7 9.5 6.0 21.7 18.2 0.044 0.030 0.047 0.035 0.046 0.041 0.082 0.089 0.096 0.089 0.096 0.073 0.037 0.033 0.038 0.041 0.039 0.165 0.088 0.104 0.159 0.101 Total-P 0.125 0.125 0.118 0.111 0.126 0.154 0.197 0.207 0.202 0.222 0.233 0.204 0.066 0.098 0.116 0.110 0.268 0.953 0.369 0.379 1.36 0.672
Lanna, clay soil, 2005/2006 A Per. ryegrass 4 Oct B Per. ryegrass 4 Oct C Per. ryegrass 14 Apr D No cover crop 4 Oct E No cover crop 4 Oct Lanne, clay soil, 2006/2007 A Per. ryegrass 4 Oct B Per. ryegrass 4 Oct C Per. ryegrass 19 Mar D No cover crop 4 Oct E No cover crop 4 Oct
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Phosphorus leaching In addition to the different cover crop and tillage practices, other factors also affected P leaching. Varying climatic conditions during the two years and soil type seemed to be of greater importance for P losses than the experimental treatments. P leaching was low from the sandy soil at Lilla Böslid (Table 1) during both years and there were no treatment effects. Chemical wilting of the cover crop in September or October resulted in rapid nutrient release from the dead plant material and there was probably an accumulation of soluble P near the soil surface similar to that after freezing (Bechmann et al., 2005), but this did not increase the risk of P losses to drainage water. This soil obviously acts as an efficient filter for P. In contrast, P leaching from the clay soil was substantial and variable between plots, probably due to preferential flow pathways (Djodjic et al., 1999). During the second year, when precipitation was high, P losses amounted to 1-1.4 kg ha-1, which can be considered high for Swedish conditions (Ulén & Jakobsson, 2005). The results clearly show the complexity behind P losses, since there were large variations between replicate plots with the same treatments. The presence of a cover crop or time of tillage (in autumn or spring) did not affect P losses at all during the first year, when drainage conditions were normal. However, during the second year, when drainage was high, soil tillage in autumn seemed to result in increased leaching of P. During winter 2006-2007, concentrations and leaching of total P in drainage water were significantly higher (P=0.023) from plots ploughed in November than from plots not ploughed at all during autumn (Figure 1). A Norwegian study on a structured soil found that soil tillage in autumn increased the transport of particles in drainage water in comparison to spring tillage (Stenrød et al., 2007). Nitrogen leaching In contrast to P leaching, N leaching was affected by the time available for cover crop growth during autumn. Chemical treatment in September or in the beginning of October on the sandy soil resulted in a rapid release of N from the plant material, increased amounts of mineral N in the soil during both years (not shown) and increased N leaching during the second year, compared with treatments where the cover crop was grown until November or until spring (P=0.012). When the cover crop was killed by chemical treatment, time of tillage (autumn or spring) did not seem to affect N leaching at all. N leaching was higher from the sandy soil than from the clay (Table 1) but during the wet year N leaching was substantial from
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both soils. N leaching from the clay soil was also significantly affected by crop cover (P= 0.037 and 0.07 during the first and second year respectively).
0.9 0.8 0.7
-1 Total-P (mg L )
Drainage water, Lanna, 2:nd year
E No cover crop, spring tillage
D No cover crop, autumn ploughing
0.6 0.5 0.4 0.3 0.2 0.1 0
Sep-06 Oct-06 Nov-06 Dec-06 Jan-07 Feb-07 Mar-07 Apr-07 May-07 Jun-07
Figure 1. Concentrations of Total-P in drainage water from the clay soil at Lanna during the wet winter of 2006-2007. Glyphosate leaching Chemical treatment during late autumn, when drainage occurs and the biological activity is low, potentially constitutes an increased risk for leaching of pesticides from many soils compared with spring or summer treatment. However, this was not verified in this study. Glyphosate was found in drainage water from all treatments on the clay soil at Lanna, both after application in autumn and in spring. This soil has a well-developed macropore system, which may favour fast transport of solutes such as glyphosate. Glyphosate was found on all sampling occasions at concentrations of 0.1-1 µg L-1, which are similar to those reported in monitoring programmes for rivers in Sweden (Adielsson et al., 2007). No glyphosate was found in drainage water from any of the treatments on the sandy soil at Lilla Böslid, not even after application of glyphosate in November. This soil obviously efficiently filters both P and glyphosate, which is reasonable since P and glyphosate are adsorbed to soil particles in the same way (mainly to Al/Fe-oxides). Conclusions The use of a cover crop did not reduce P losses from a clay or a sandy soil. The time of chemical treatment (i.e. glyphosate) also had no effect on P losses to drainage water. P leaching from the sandy soil was very low overall, while it was more variable and higher from the clay soil. During one wet year, P losses were considerable and autumn tillage gave higher P losses than spring tillage. However,
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spring tillage on the clay soil resulted in problems with seedbed preparation and cannot be recommended on basis of these results. The use of a cover crop reduced N leaching, while delaying the time of chemical treatment, i.e. leaving more time available for cover crop growth, clearly decreased N leaching on the sandy soil. The time of tillage after chemical treatment seemed to be of minor importance for N leaching. The results show that cover crops are important for reducing N leaching from sandy soils without increasing the risk of glyphosate leaching (if used for wilting the cover crop). On structured clay soils such as that at Lanna, there is always a risk of glyphosate leaching and on these soils, cropping systems with frequent use of glyphosate may carry a risk of negative impacts on the environment. References Adielsson, S., Törnquist, M., Kreuger, J., 2007. Bekämpningsmedel i vatten och sediment från typområden och åar samt i nederbörs under 2007. Ekohydrologi 99. Avdelningen för vattenvårdslära, SLU, Uppsala. Bechmann, M., Kleinman, P.J.A., Sharpley, A.N., Saporito, L., 2005. Effect of freezing and thawing on fate of phosphorus in bare, manured and catch cropped soils. J. Environ. Qual. 34, 2301-2309. Djodjic, F., Bergström, L., Ulén, B., Shirmohammadi, A., 1999. Mode of transport of surface applied P-33 through a clay and sandy soil. J. Environ. Qual. 28, 12721282. Stenrød, M., Ludvigsen, G.H., Riise, G., Lundekvam, H., Almvik, M., Tørresen, K.S., Øygarden, L., 2007. Redusert jordarbeiding og glyfosat. Bioforsk Rapport Vol.2 Nr. 145 2007. Ulén, B., Jakobsson, C., 2005. Critical evaluations of measures to mitigate phosphorus losses from agricultural land to surface waters in Sweden. Sci. Total Environ. 344, 37-50.
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