Volume 114, June 2017, Pages 105–110

Short communication

Increased plant uptake of native soil nitrogen following fertilizer addition – not a priming effect?

  • a Center for Ecosystem Science and Society (Ecoss) and Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, USA
  • b Department of Microbiology, University of Massachusetts, Amherst, MA 01003, USA
  • c Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4 RJ, UK

Highlights

Organic fertilizer tended to reduce but inorganic fertilizer tended to increase mineralization of native soil N.

Organic and inorganic fertilizers increased plant uptake of native soil N.

Organic fertilizer showed contrasting effects on plant uptake and mineralization of native soil N.

Increased plant uptake of native soil N mostly controlled by plant- rather than microbe- mediated mechanisms.


Abstract

Fertilizer inputs affect plant uptake of native soil nitrogen (N), yet the underlying mechanisms remain elusive. To increase mechanistic insight into this phenomenon, we evaluated the effect of fertilizer addition on mineralization (in the absence of plants) and plant uptake of native soil N. We synthesized 43 isotope tracer (15N) studies and estimated the effects of fertilizer addition using meta-analysis. We found that organic fertilizer tended to reduce native soil N mineralization (−99 kg ha−1 year−1; p = 0.09) while inorganic fertilizer tended to increase N priming (58 kg ha−1 year−1; p = 0.17). In contrast, both organic and inorganic fertilizers significantly increased plant uptake of native soil N (179 and 107 kg ha−1 year−1). Organic fertilizer had greater effect on plant uptake than on mineralization of native soil N (p < 0.001), but inorganic fertilizer had similar effects. Fertilizer effects on mineralization and plant uptake of native soil N were not influenced by study location (laboratory or field) and duration, soil texture, carbon and N content, and pH. Fertilizer addition variably affected native soil N mineralization but consistently increased plant uptake of native soil N. The positive effect of organic fertilizer on plant uptake of native soil N can not be explained by its negative effect on native soil N mineralization, suggesting that increased plant uptake of native soil N was caused mostly by plant-mediated mechanisms (e.g., increased root growth, rhizosphere N priming) rather than by soil microbe-mediated mechanisms.

Keywords

  • 15N isotope tracer;
  • Added nitrogen interaction;
  • Mineralization and immobilization;
  • 15N labeled fertilizer;
  • Plant-soil interaction;
  • Rhizosphere priming

1. Introduction

Plants often grow faster with supplemental nitrogen (N), which is applied to agricultural fields as fertilizer (Asagi and Ueno, 2009 ;  Bosshard et al., 2009) or enters natural ecosystems in the form of atmospheric deposition (Reich et al., 2001 ;  Schimel and Bennett, 2004). Although this added N directly increases N availability and plant growth, it can also affect the release of N from soil organic matter (SOM) (hereafter, “native soil N”).

Nitrogen priming (or “added N interaction” in older literature) is defined as the increase or decrease in the amount of native soil N released by microbial mineralization or taken up by plants with fertilizer addition compared to treatments without added N (Jenkinson et al., 1985). Priming is best measured using 15N-enriched fertilizers to distinguish added N from the unlabeled native soil N (Barraclough, 1995). Plant uptake of native soil N can be boosted either by the increase in native bulk soil N mineralization or by plant-mediated processes, such as increased root growth and rhizosphere N priming (Ashraf et al., 2004; Jenkinson et al., 1985 ;  Schimel and Bennett, 2004). Native soil N priming dynamics are thought to be affected by soil type, fertilizer type, and environmental factors (Ghaley et al., 2010; Glendining et al., 1997; Hgaza et al., 2012; Liu et al., 2017; Recous et al., 1988; Rowlings et al., 2016 ;  Westerman and Kurtz, 1973). However, it remains unclear what mechanisms regulate native soil N dynamics following fertilizer inputs. Hence, we used meta-analysis to assess the effect of fertilizer addition on mineralization and plant uptake of native soil N, testing for broad patterns across experiments performed to date.

2. Materials and methods

2.1. Data collection

We used the ISI Web of Science and Google Scholar to exhaustively search papers published before March 2017. Several terms (“soil nitrogen priming”, “nitrogen 15”, “soil 15N”, “added nitrogen interaction”, “labeled 15N fertilizer”, “nitrogen fertilization”, and “nitrogen mineralization”) were used to identify studies that compared mineralization or plant uptake of native soil N in control (no-fertilizer applied) and fertilizer treatments. To be included in our dataset, studies were required to have used 15N isotope tracers to separate unlabeled native soil N from labeled fertilizer N, and to report data for both fertilizer and control plots. We tabulated studies where organic fertilizers (e.g., animal manure, compost, crop residues) or inorganic (chemical) fertilizers were added. Study location (laboratory or field) and duration, soil texture (sand, silt, or clay), carbon and N content, and pH were tabulated when available. Soils classified as silty loams were included in the silt category, clayey loams in the clay category, and sandy loams in the sand category. Our dataset did not include studies on soils that were classified as loam. For studies with multiple observations over time (e.g., multiple growing seasons or years), data were averaged across time to avoid the temporal pseudoreplication. Our final dataset included 43 studies that were conducted on all continents except Antarctica (Fig. 1), involving a wide range of plant species, fertilizer types, and soil physicochemical characteristics (Table 1 ;  Table 2).