Method and timing of grassland renovation affects

Authors: G L Velthof I E Hoving J Dolfing A Smit P J Kuikman O Oenema

Publish Date: 2009/08/15

Volume: 86, Issue: 3, Pages: 401-412

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Abstract

Managed grasslands are occasionally ploughed up and reseeded in order to maintain or increase the sward productivity It has been reported that this renovation of grassland is associated with a flush of soil organic nitrogen N mineralization and with a temporary increase in soil mineral N contents Here we report on the effects of method and time of grassland renovation on herbage yield nitrate NO3 − leaching and nitrous oxide N2O emission Field experiments were carried out at three sites two sandy soils and a clay soil in the Netherlands for three years Renovation of grassland increased the percentage of Perennial ryegrass from 48–70 up to more than 90 However averaged over three years dry matter yields were higher for the reference not reseeded swards on average 136 Mg ha−1 for the highest N application rate than for the renovated grasslands 122–131 Mg ha−1 dry matter Grassland renovation in April did not increase N leaching in comparison to the reference However renovation in September increased the risk of leaching because mineral N contents in the 0–90 cm were in November on average 46–77 kg N ha−1 higher than in the reference Contents of dissolved organic N DON in the soil were not affected by renovation Renovation increased N2O emissions by a factor of 18–30 relative to the reference grassland Emissions of N2O were on average higher after renovation in April 82 kg N2ON ha−1 than in September 58 kg N2ON ha−1 Renovation without ploughing ie only chemically destruction of the sward resulted in a lower percentage of perennial ryegrass 60–84 than with ploughing 90 Moreover N2O emissions were higher after renovation without ploughing than with ploughing Clearly farmers need better recommendations and tools for determining when grassland renovation has beneficial agronomic effects Losses of N via leaching and N2O emission after renovation can probably not be avoided but renovation in spring in stead of autumn in combination with ploughing and proper timing of fertilizer application can minimize N lossesPermanent grasslands are important sources of feed in the intensively managed dairy and beef farming systems in North West Europe These grasslands are harvested by grazing and cutting for making silage and hay and for feeding zerograzed cattle Mean grassland yields roughly range between 6 and 14 Mg ha−1 year−1 depending on climate management and fertilization Whitehead 2000 It has been often observed in practice that yields and quality of permanent grasslands decrease during ageing because of sward deterioration following extreme weather conditions and/or bad management As a consequence many farmers occasionally plough up and reseed these intensively managed grasslands in order to maintain or increase the sward productivity In the Netherlands intensively managed grasslands on sandy soils are reseeded on average every five years and those on clayey soils every ten years Schils et al 2002 Grassland renovation may both increase the percentage of perennial ryegrass in the sward and enhance soil conditions eg better aeration by ploughing Reseeding in the second half of the growing season is preferred by most farmers because the chance of a successful establishment of the sward with low weed infestation is high in this relatively warm and moist period Moreover farmers consider loosing a period of grass production in autumn less of a problem than loosing the period of relatively high grass production in springNitrogen N inputs via N fertilizer and animal manure to intensively managed grasslands are relatively high 200 kg N ha−1 Bos et al 2005 Grassland has a relatively high N uptake capacity both in aboveground and belowground biomass A fraction of the annual N input accumulates in stubbles roots and microbial biomass and ends up in dead soil organic matter during ageing of grassland In living roots N accumulates over time to more than 200 kg N ha−1 however the rate of N accumulation strongly levels off during ageing of grasslands Eriksen and Jensen 2001 Whitehead et al 1990 Stubbles of less than five years contain about 40–80 kg N ha−1 and this amount does not significantly increase with ageing Davies et al 2001 Whitehead et al 1990 The N content of the soil organic matter of grassland increases approximately linearly in young grasslands 10 years ranging from 20 to 130 kg N ha−1 year−1 Cuttle and Scholefield 1995 Hassink 1994 Tyson et al 1990 Whitehead et al 1990 The accumulation of organic matter levels off during ageing and may continue for more than 30–100 years Jenkinson 1988 The C/N ratio of the soil organic matter of intensively managed grasslands is commonly in the range of 12–15 but depends on soil type Whitehead 2000Because of the high N content of grassland soils renovation may temporary increase the content of mineral N to more than 200 kg N ha−1 Davies et al 2001 Johnston et al 1994 Lloyd 1992 Shepherd et al 2001 This increase has been related to three factors Firstly stubbles and roots of the grass sward are ploughed into the soil and the net mineralization of these crop residues increases soil mineral N contents Secondly there is a period with no or only small crop uptake of N after ploughing This period ranges from several weeks in case of immediately reseeding grassland to several months in case grassland is ploughed in autumn and the following crop is grown in next spring The third factor is a possible enhanced N mineralization of soil organic nitrogen because of ploughing This enhanced N mineralization is in part related also to the relatively low C/N ratioThe risks of increased N leaching and nitrous oxide N2O emission are relatively high in case the periods with high soil mineral N contents coincide with wet climatic conditions For example it has been reported that leaving the soil fallow after ploughing of grassland resulted in leaching losses of 100–300 kg N ha−1 year−1 Adams and Jan 1999 Davies et al 2001 Lloyd 1992 Bhogal et al 2000 showed that significant leaching of dissolved organic N DON may occur after ploughing of grassland Leaching of DON can be a significant N loss pathway from agriculture Van Kessel et al 2009 Davies et al 2001 showed that ploughing of grassland increased N2O emission for several weeks and total N2O emissions from ploughed grassland ranged from 15 to 37 kg N ha−1 year−1 Evidently ploughing up and reseeding grasslands is associated with significantly increased N losses to surrounding surface waters groundwater and air but the empirical evidence for recommending reseeding methods and practices with minimum N losses are as yet lacking

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