Potential to improve nitrogen use efficiency (NUE) by use of perennial mobile green manures

Nutr Cycl Agroecosyst https://doi.org/10.1007/s10705-022-10253-x ORIGINAL ARTICLE Potential to improve nitrogen use efficiency (NUE) by use of perennial mobile green manures C. R. Ward · D. R. Chadwick · P. W. Hill Received: 16 December 2021 / Accepted: 28 November 2022 © The Author(s) 2022 Abstract Supplying nitrogen (N) to crops by incorporating N2-fixing green manures into soil can improve soil functioning and increase soil carbon storage. However, as with N-fertiliser use, excess mineral N from decomposed green manures can be lost via leaching as NO3− and emitted as the greenhouse gas N2O. To improve nitrogen use efficiency (NUE) we propose supplying leaf matter harvested from permanent stands of N2-fixing plants grown ex-situ as perennial mobile green manures (PMGMs), thus allowing flexible timing of additions to synchronise with N demand. To assess the effectivity of PMGM use, we monitored crop N uptake, biomass production and soil mineral N from fertilisation by three potential PMGM species, Alnus glutinosa (alder), Gunnera tinctoria (gunnera), and Ulex europaeus (gorse), compared to the conventional green manure Trifolium pratense (red clover) and N H4NO3 fertiliser. Effectivity of N fertilisation by PMGMs increased with the duration of their use. In the first season of a field experiment, crop N uptake after addition of 200 kg N ha−1 within gunnera was equivalent to that Supplementary Information The online version contains supplementary material available at https://doi. org/10.1007/s10705-022-10253-x. C. R. Ward (*) · D. R. Chadwick · P. W. Hill School of Environment, Natural Resources and Geography, Environment Centre Wales, Bangor University, Bangor, Gwynedd LL57 2UW, UK e-mail: from only 14 kg N H4NO3-N ha−1, but after two yearly harvests with annual gunnera additions it increased to 70 kg NH4NO3-N ha−1 year−1 equivalent. In a 1-year pot experiment, PMGMs resulted in equal or higher crop biomass than clover-fed crops, with lower soil NO3− (≤ 25 mg N L−1 soil solution) than in clovertreated soil (≥ 130 mg N L−1). We conclude that PMGMs have potential to increase NUE over that of traditional green manures and discuss the feasibility and possible agro-ecological benefits of PMGM-fed systems. Keywords NUE · Soil nitrogen · Perennials · Green manure · Cover crops · Sustainable agriculture Introduction Today’s high agricultural yields are made possible by supplying crops with additional plant-available N, which has been converted from atmospheric N 2 by industrial N2 fixing or by the growing of N 2-fixing leguminous plants (Battye et al. 2017). This has resulted in unprecedented amounts of reactive N in agricultural systems, which is environmentally detrimental regardless of the method of N 2 fixation (Rockstrom et al. 2009; Smil 2011; Fowler et al. 2013). Nitrogen fertiliser production accounts for 1–2% of the world’s fossil fuel use, causing associated CO2 emissions (Houlton et al. 2019). In contrast, biological N2 fixing does not cause CO2 emissions, Vol.: (0123456789) 13 Nutr Cycl Agroecosyst and addition of plant matter to soil can increase soil C storage (Lehtinen et al. 2014; McClelland et al. 2021) and improve soil health (Kibblewhite et al. 2008). Biologically fixed N 2 is usually supplied via leguminous green manures such as clovers or vetches which are grown on agricultural land and incorporated in situ into soil (Blanco-Canqui et al. 2015; Carr et al. 2020). As the plant matter decomposes, N becomes available for uptake by the following crop as ammonium (NH4+), nitrate (NO3−), and small organic N molecules (Farzadfar et al. 2021; Udvardi et al. 2021). Bacterial N2 fixing, however, powered by photosynthetically fixed C, necessitates periodically setting aside cropland for sunlight energy capture, which reduces the overall yields of the cropping system (Crews and Peoples 2004; Powlson et al. 2011; Smith et al. 2018). Nitrogen losses can occur after the addition of fertiliser or green manure N by leaching of NO3− (Neeteson and Carton 2001; Di and Cameron 2002; Campiglia et al. 2011) and dissolved organic N (van Kessel et al. 2009) and by ammonia volatilisation (de Ruijter et al. 2010; Nett et al. 2016; Pan et al. 2016). Emission of nitrous oxide (N2O) can also occur, principally by nitrification of N H4+ or denitrification of − NO3 (Chen et al. 2013; Butterbach-Bahl et al. 2013; Rees et al. 2013). Global N2O emissions, the majority of which are from agricultural soils, account for 3.4% of total anthropogenic greenhouse gas (GHG) emissions, and 10% of the climate change impact of global food production (Crippa et al. 2021). Nitrogen losses increase with higher soil mineral N concentrations which occur when N supply exceeds crop demand. To reduce losses, fertilisers are commonly applied to crops in two or more (split) applications, and precision agriculture techniques are increasingly used to match supply with crop demand (Norton and Ouyang 2019; Tei et al. 2020; Rees et al. 2020). The incorporation of green manures into soil, however, offers less scope to synchronise N supply with crop demand (Cassman et al. 2002; Crews and Peoples 2005; Finney et al. 2015; Mὃller 2018). To better match the supply of green manure N with crop demand, some growers use leaf material harvested from green manures grown outside the cropping area and added to arable land when required (Crews and Peoples 2005; Mὃller and Stinner 2009; Carter et al. 2014). For example, Burgt et al. (2018) trialled grass-legume and grass mixes, applied fresh Vol:. (1234567890) 13 or pelleted, and Sorensen and Thorup-Kristensen (2011) trialled various plant species as “mobile green manures” to fertilise vegetable crops. Similarly, in tropical agroforestry, soil is fertilised with green prunings from N 2-fixing trees (Palm 1995; Ajayi et al. 2011; Sileshi et al. 2020). Here we propose a system of green manuring for temperate agriculture in which N is fixed by perennials including trees and shrubs grown ex-situ in multispecies agroforestry areas, or “bio-service areas.” As far as we are aware, such a system is new to temperate agriculture, and we refer to plants used in this way as perennial mobile green manures (PMGMs). Use of species suited to growing in exposed or waterlogged conditions, allows the “bio-service areas” to be situated on marginal land, so reducing requirement for prime agricultural land for N2-fixation. To synchronise N additions with crop demand, knowledge is needed on the rate at which N becomes available after the PMGMs are added to soil. This depends on the interaction of leaf characteristics with the physical and chemical conditions of the soil, such as pH and temperature. Nitrogen will typically be immobilised and unavailable to crops if the C:N ratio of the substrate is ≥ 25:1 (Robertson and Groffman 2007; Farzadfar et al. 2021). Over time, however, substrate C is utilised by microbes to a point where microbes become C limited and N becomes available (...truncated)