Short-term competition between crop plants and soil microbes for inorganic N fertilizer

Abstract

Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH₄⁺), nitrate (NO₃⁻) or a combination thereof are expected to differ in soil N transformation rates and fates of NH₄⁺ and NO₃⁻. Using ¹⁵N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH₄⁺, NO₃⁻ or NH₄NO₃. Within each fertilizer treatment traces of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were added separately. During 8 days of fertilization the fate of fertilizer ¹⁵N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45–80% of initially applied ¹⁵N was recovered in crop plants compared to only 1–10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO₃⁻ than NH₄⁺ independent of initially applied N form. Surprisingly, no inhibitory effect of NH₄⁺ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH₄⁺ uptake rates, gross nitrification rates were 3–75-fold higher, indicating that nitrifiers were the strongest competitors for NH₄⁺ in both soils. The rapid conversion of NH₄⁺ to NO₃⁻ and preferential use of NO₃⁻ by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO₃⁻ and shift towards enhanced reliance on NO₃⁻ for their N supply.

Introduction

Intensive agricultural crop production in central Europe largely depends on the input of nitrogen fertilizers, mainly provided in the form of NH₄⁺ or NO₃⁻ or a combination thereof. Global estimates indicate that less than ∼50% of the applied fertilizer N are used by the crop, while 2–5% are stored in the soil, ∼25% are emitted to the atmosphere and ∼20% are discharged to aquatic systems (Galloway et al., 2004). In the last decade a wealth of studies have focused on ways to improve fertilizer use efficiency, largely in cereal grain production, and on reducing adverse effects, namely losses of fertilizer N to the environment (Tilman et al., 2002, Mosier et al., 2004). Other studies have focused on fates of N inputs during one or more growing seasons, or on predicting N availability to crops (Jackson et al., 2008).

In agricultural soils plant available N is present in soluble inorganic (NO₃⁻, NH₄⁺, NO₂⁻) and organic N forms. The pool of soluble organic N in unfertilized arable soils can be as large as the mineral N pool and might be strongly involved in mineralization and immobilization processes (reviewed by Murphy et al., 2000). Although dissolved organic N, especially free amino acids, may contribute significantly to plant nutrition in several ecosystems (reviewed by Näsholm et al., 2009), the quantitative importance seems to be negligible (<1%) in agricultural systems that receive high quantities of inorganic N fertilizer (Xu et al., 2008). This indicates that the plant N relations in agricultural soils are predominantly influenced by the rate of mineral N fertilizer application. But not only the amount, also the form of mineral N applied (i.e. NO₃⁻ vs. NH₄⁺ vs. NH₄NO₃) is strongly affecting plant and microbial N metabolism.

In many cultivated soils it has been observed that in plants and microbes NH₄⁺ assimilation exceeds NO₃⁻ assimilation (e.g. Azam et al., 1993), which was explained by higher energy costs associated with biological NO₃⁻ assimilation (Gutschick, 1981, Smirnoff and Stewart, 1985, Puri and Ashman, 1999). Moreover, it was shown that the presence of NH₄⁺ inhibits NO₃⁻ uptake by fungi (Wang et al., 2007) and by plants (Gessler et al., 1998, Gazzarrini et al., 1999, Siddiqi et al., 2002), further promoting plant NH₄⁺ uptake. However, in contrast to these molecular studies, other studies using soils showed in this environment that NH₄⁺ utilization by soil microbes and plants was lower or similar to NO₃⁻ utilization (Burger and Jackson, 2003, Song et al., 2007), presumably due to higher mobility of NO₃⁻ in soils as compared to NH₄⁺ (Hodge et al., 2000).

One factor leading to these differences in inorganic N assimilation might be that the inorganic N pools in agricultural soils are extremely dynamic (e.g. Jackson et al., 1989), caused by high microbial activities and fertilizer application. Although these dynamics are well established, little is still known about the mechanisms and short-term effects (hours to days) of different ratios of NH₄⁺:NO₃⁻ on microbial N transformation processes. As some of these processes lead to N losses to the environment (e.g. leaching of NO₃⁻ or N gas emissions caused by nitrification or denitrification), even short-term increases in microbial activity can negatively affect the fertilizer use efficiency of crop plants.

Besides the availability of inorganic N, competition with soil microbes is one of the most critical factors affecting the ability of plants to acquire N from soil (Kaye and Hart, 1997). In several short-term ¹⁵N tracer studies (up to several days) investigating grassland ecosystems, microbes often assimilated more ¹⁵N-labelled inorganic N than plants did, but after an initially rapid N capture, microbial biomass appeared to reach a steady state, probably because of insufficient available C to maintain the fast initial growth rates (Hodge et al., 2000, Harrison et al., 2008). The same studies showed that during the following period, microbial ¹⁵N was gradually released by microbial decay and remineralization into the soil, eventually becoming available for plant root uptake. After longer time periods (weeks to months), plants contained an increasing proportion of the added ¹⁵N, showing that plants indeed utilized this N resource (Harrison et al., 2007). In other words, the turnover rate of plant biomass was much slower than that of microbes, which allowed plants to compete for the same N for extended periods, therefore enhancing plant competitiveness for N.

However, in high N ecosystems, like in agricultural soils, it was suggested that competition could be less severe (Schimel and Bennett, 2004), though plants and microbes are considered to compete for any available N, particularly for NH₄⁺ and NO₃⁻. Further, besides competition between plants and soil microbes, competition among different groups of soil microbes for fertilizer N is of considerable importance. Burger and Jackson (2003) found that, due to the high availability of inorganic N after fertilization, competition between plants and heterotrophic microbes for NH₄⁺ markedly decreased and nitrification became the major fate of NH₄⁺. Over time, the N economy in well-aerated agricultural soils therefore becomes progressively NO₃⁻ dominated and plants cover their N-demand from mainly NO₃⁻ (Schimel and Bennett, 2004). However, it still remains unclear to which degree short-term (hours to days) competition for fertilizer N between plants, heterotrophic and autotrophic microbes is affected by the form and amount of mineral N applied.

The main objectives of the present study therefore were to assess (1) the effect of applied inorganic N form on the degree of competition for fertilizer N between crop plants and soil microbes, (2) the population response of bacteria and fungi to the different N treatments, as determined by estimation of gene copy numbers (quantitative PCR), (3) preferences in uptake of NH₄⁺ or NO₃⁻ by plant roots and soil microbes as well as possible inhibitory effects of one N form on uptake of the other form and (4) fertilizer induced changes of microbially mediated N transformation rates that control N distribution to the different soil compartments as well as losses of fertilizer N to the environment.