Plant litter variability and soil N mobility Hongtao Zhong A , Carol Smith B , Brett Robinson B , Young-Nam Kim A , and Nicholas Dickinson A,C A Department of Ecology, Lincoln University, Lincoln 7647, Canterbury, New Zealand. B Department of Soil and Physical Sciences, Lincoln University, Lincoln 7647, Canterbury, New Zealand. C Corresponding author. Email: nicholas.dickinson@lincoln.ac.nz Abstract. Laboratory incubation studies were used to investigate whether and how variability of different plant litters modifies the mobility of nitrogen in soil. Fallen plant foliage from native New Zealand plants of diverse fibre and nutrient content were selected, with C : N ratios ranging from 14 to 102. Different litters provided substantially different inputs of macro- and micronutrients to soil that affected the mobility of N. Both fibre content and C : N ratios were influential. A primary effect of litter addition to soil was modification of pH, largely attributable to calcium enrichment. Nitrate in soil was reduced by up to 85% following litter amendments. Incorporation of five native plant litters into soil significantly suppressed emissions of nitrous oxide. We interpret these findings in the context of plant residues from naturalistic planting on the borders of farm paddocks that may play a role in tightening the N cycle and restricting spillover of nitrogen pollutants to the wider environment. Additional keywords: agrobiodiversity, agroecology, nitrate, nitrous oxide, soil amelioration. Received 18 May 2016, accepted 18 September 2016, published online 8 November 2016 Introduction Plant litter input and its decomposition modifies the physical, chemical and biological properties of soil, protecting soil from erosion (Marshall et al. 1996; Li et al. 2014), mediating soil temperature and water content (Judas 1990; Ogée and Brunet 2002) and providing a major source of organic matter and nutrients to soil (Hobbie 2015). Litter fall is also a determinate factor in the assemblages of invertebrates and microbial communities found in soil (Sayer 2006). The value of litter has been viewed largely in terms of provisioning soil development and nutrient cycling, in turn contributing to ecosystem health (Bardgett and van der Putten 2014). Much less attention has been given to the role of litter variability in containing the mineralisation of nutrients and limiting the release of fractions of nitrogen pollutants to the wider environment. Litter decomposition processes are driven by multiple factors, such as litter quality, substrate characteristics and soil decomposer communities (Berg and McClaugherty 2008). Relatively small differences in chemical composition of foliar amendments are known to make large differences in N and P mineralisation, accumulation and depletion in soil (Constantinides and Fownes 1994; Kumar and Goh 2003; Alamgir et al. 2012; Damon et al. 2014). Furthermore, amending soils with fresh plant material (including ryegrass) as green manure may enhance nitrous oxide (N 2 O) emissions (Baggs et al. 2000; Mitchell et al. 2013; Zhu et al. 2013). N 2 O emissions have been correlated with CO 2 flux in various plant residues, but negatively correlated with the C : N ratio of residues and recalcitrant carbon compounds (Huang et al. 2004; Millar and Baggs 2004; Yanni et al. 2011). It appears to be difficult to predict the magnitude and direction of N 2 O emissions, but organic amendments stimulate microbial respiration, thus depleting O 2 and increasing anaerobic conditions for denitrification (Miller et al. 2008; Chen et al. 2013). It has been suggested that N 2 O losses to the atmosphere could be reduced by applying different combinations of plant materials or N fertiliser of different qualities and ages to soil (Millar and Baggs 2004; Gentile et al. 2008). Recycling organic wastes to soil has obvious benefits to sustainable soil management practices in agroecosystems, but research has largely focused on agricultural and human wastes rather than natural vegetation. Organic amendments to soil potentially have some disadvantages, such as nitrogen losses to the wider environment through both enhanced nitrate (NO 3 – ) leaching to water bodies and gaseous emissions of N 2 O (Thangarajan et al. 2013). In New Zealand, agriculture is now the largest anthropogenic source of NO 3 – and N 2 O, contributing 46.5% of total emissions of the greenhouse gas (GHG) profile, the highest for any developed country (Thorburn et al. 2012). Tightening the N cycle is vital for the future of integrated soil fertility management of farmland (Yanni et al. 2011). In the present study, we investigated (1) the variability of leaf litter chemistry between New Zealand native plant species and (2) how native plant litters modify marginal farmland soil chemistry and potentially contribute to the amelioration of significant environmental concerns around nitrate leaching and GHG emissions. We questioned whether native plant litters Journal compilation Ó CSIRO 2017 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2017, 55, 253–263 http://dx.doi.org/10.1071/SR16132