Phosphorus addition enhances loss of nitrogen in a phosphorus-poor soil Mingzhu He a, * , Feike A. Dijkstra b a Shapotou Desert Research and Experiment Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, 730000, China b Department of Environmental Sciences, The University of Sydney, Centre for Carbon, Water and Food, 380 Werombi Road, Camden NSW, 2570, Australia article info Article history: Received 13 August 2014 Received in revised form 22 December 2014 Accepted 24 December 2014 Available online 5 January 2015 Keywords: 15 N labelling Denitrification Nitrification N/P stoichiometry N and P cycling Soil moisture abstract Plants and microbes have limited stoichiometric flexibility to take up and store nitrogen (N) and phosphorus (P). Variation in the relative availability of N and P to plants and microbes may therefore affect how strongly N and P are held in terrestrial ecosystems with important implications for net pri- mary productivity and carbon sequestration. We hypothesized that an increase in P availability in a P- poor soil would increase N uptake by plants and microbes thereby reducing N loss. We grew mixtures of the C3 grass Phalaris aquatica L. and the legume Medicago sativa L. in mesocosms with soils low in P availability and then used a novel technique by adding a 15 N tracer with and without 1 g P m 2 to soil with different moisture and available N conditions, and measured the 15 N recovery after 48 h in mi- crobes, plants and soil. In contrast to our hypothesis, we found that P addition reduced 15 N in microbes without water stress by 80% and also reduced total 15 N recovery, particularly without water stress. Water stress in combination with N addition further showed low total 15 N recovery, possibly because of reduced plant uptake thereby leaving more 15 N in the soil available for nitrification and denitrification. Our results suggest that P addition can result in large gaseous N loss in P-poor soils, most likely by directly stim- ulating nitrification and denitrification. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Nitrogen (N) and phosphorus (P) are important nutrients frequently limiting plant growth in terrestrial ecosystems (Elser et al., 2007; Harpole et al., 2011). Increased atmospheric N depo- sition caused by fuel combustion, agriculture and other human activities, has resulted in greater availability of N relative to P in many ecosystems worldwide affecting ecosystem structure, func- tioning and diversity (Vitousek et al., 1997; Galloway et al., 2008; Pe~ nuelas et al., 2013). With dwindling supplies of phosphate rocks needed to manufacture P fertilizers, the imbalanced avail- ability of N and P (i.e., reduced P availability compared to N) has become particularly critical in agricultural systems with important consequences for food security (Pe~ nuelas et al., 2013; van der Velde et al., 2014). Excess of available N compared to P could enhance gaseous N loss through microbially-mediated processes of nitrifi- cation and denitrification (Vitousek et al., 1997; Hall and Matson, 1999; Stehfest and Bouwman, 2006) that are often limited by N (Bouwman et al., 2002). Because of N/P stoichiometric constraints of plants and microbes, excess of available N compared to P may enhance P limitation, which could affect N retention and loss in different ways than when plants and microbes are N limited. Plant growth is frequently constrained by P availability (Vitousek et al., 2010), but soil microbial activity can also show P limitation. Microbes, more so than plants, have a high P require- ment compared to N. Microbial N/P ratios were lower than plant N/ P and soil N/P across a wide variety of ecosystems (Cleveland and Liptzin, 2007), suggesting that microbial activity may be more constrained by P than plant activity. The requirement for P may increase relative to N with increased microbial activity because of the greater need of P-rich RNA to synthesize proteins (Elser et al., 1996; Franklin et al., 2011). Microbial activity and growth may therefore particularly be constrained by P when activity is high. Microbial P limitation has mostly been observed in highly weath- ered tropical soils (Cleveland et al., 2002; Ehlers et al., 2010), but also in calcareous (Raiesi and Ghollarata, 2006), peat (Hill et al., 2014), and boreal forest soils (Giesler et al., 2002). * Corresponding author. Tel.: þ86 931 4967193. E-mail addresses: hmzecology@lzb.ac.cn (M. He), feike.dijkstra@sydney.edu.au (F.A. Dijkstra). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio http://dx.doi.org/10.1016/j.soilbio.2014.12.015 0038-0717/© 2015 Elsevier Ltd. All rights reserved. Soil Biology & Biochemistry 82 (2015) 99e106