Bovine urine inhibits microbial function and increases urea turnover in dairy grazed soils S. M. Lambie A,B , N. W. H. Mason A , and P. L. Mudge A A Manaaki Whenua – Landcare Research, Private Bag 3127, Hamilton, New Zealand. B Corresponding author. Email: lambies@landcareresearch.co.nz Abstract. Effects of bovine urine on microbial functional attributes within the carbon (C) cycle have not previously been investigated. The magnitude of urine effects on microbial populations may be mediated by the ability of a soil to buffer changes to pH and electrical conductivity (EC) in response to urine. We examined changes in the metabolism of C substrates by microbial communities subsequent to treatment with dairy cow urine in 27 dairy grazed soils across four soil orders. Untreated soils (baseline) and soil treated with urine or water were incubated (258C) for 21 days then assessed for microbial function using MicroRespÔ. Urine addition decreased functional capacity, microbial diversity, and microbial biomass C at 21 days after urine addition, but did not affect basal respiration, compared with the water control. Urine addition also led to a shift in community-level physiological profiles. There were no indirect effects of soil pH or EC buffering capacity on the functional microbial parameters measured. Urine addition increased the utilisation of urea and may be a factor in losses of fertiliser nitrogen in dairy systems. The length of time that urine depresses catabolic function could have important implications for long-term soil organic matter cycling under urine patches. Additional keywords: community-level physiological profiles, functional capacity, functional diversity, MicroRespÔ; multiple substrate-induced respiration. Received 28 August 2018, accepted 21 March 2019, published online 30 May 2019 Introduction Microorganisms play a pivotal role in delivering ecosystem services including carbon (C) sequestration and nutrient provision (Aislabie and Deslippe 2013). Losses of microbial function and diversity can indicate a decrease in soil quality (Bending et al. 2000; Chapman et al. 2007) and may compromise the ability of a soil to deliver ecosystem services (Dominati 2013). Bovine urine alters microbial functioning within the nitrogen (N) cycle. For example, Orwin et al.(2010) suggested that nitrite oxidisers may be compromised in urine patches due to high salt contents. Urine also alters microbial community structure by increasing the proportion of Firmicutes which may contribute to changes in microbial function (O’Callaghan et al. 2010). How urine alters C cycling is largely unknown although Lambie et al. (2012a, 2012b, 2013) showed that urine addition can lead to priming of soil C decomposition, as well as solubilisation and leaching of soil C. However, further assessment of whether urine addition affects the degradation of added substrates was still required to determine the effect of urine additions of C cycling. In theory, urine could enhance heterotrophic processes by introducing C substrates (Lambie et al. 2013), but also create a hostile environment for microbes (Bertram et al. 2012) which may negate any positive effects of urine addition. Soil pH is an important factor affecting microbial community composition (e.g. Fierer and Jackson 2006) and electrical conductivity (EC) in urine patches has been shown to impede microbial function (Orwin et al. 2010). Soil type is also an important factor affecting microbial function (Girvan et al. 2003; Wakelin et al. 2008; Berg and Smalla 2009); however, research investigating the microbial response to urineaddition has been limited to a small number of soils and it is not clear whether urine-derived effects on function are consistent within and between soil types. Further, microbial responses to urine addition may be confounded by changes to soil pH and EC subsequent to urine addition (Lambie 2012), which may differ among soils (Magdott and Bartlett 1984; Curtin and Rostad 1997; Weaver et al. 2004). The extent to which urine additions alter soil pH and EC in different soils has not been established and could influence the extent of microbial response to urine addition. Multiple substrate-induced respiration techniques have been used successfully to characterise microbial responses to perturbations (e.g. Degens et al. 2000; Govaerets et al. 2007; Fierer et al. 2012; Bérard et al. 2014) and therefore we used MicroRespÔ to assess functional capacity, functional diversity and community-level physiological profiles (CLPPs) in response to urine addition. To assess differences in these microbial parameters between soils with potentially different buffering capacities, we selected 27 soils from four soil orders. We tested the hypothesis that urine addition decreases soil functional capacity and functional diversity and shifts CLPPs. Further, we hypothesised that microbial functional parameters would be indirectly affected by differences in soil buffering capacity of soil pH and EC subsequent to urine addition. Journal compilation Ó CSIRO 2019 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research https://doi.org/10.1071/SR18257