Nitrogen fertilization decreases the decomposition of soil organic matter and plant residues in planted soils

Highlights

• Urea fertilization slowed the decomposition rate of light fraction N and organic C.

• N fertilization slowed decomposition of both maize residues and soil organic matter.

• The decreased mineral N level unaffected microbial biomass and N use.

• In soils limited by mineral N microbes used N originated more from organic sources.

Abstract

Nitrogen fertilization may affect the decomposition of soil organic matter (SOM) and plant residues in soil, but this effect is still very uncertain and depends on living plants. We investigated the effects of mineral N (N_min) availability on SOM and plant residue decomposition in wheat (Triticum aestivum L.) growing soils in a pot experiment. Five treatments were assessed: (1) Control [no maize (Zea Mays L.) residues and no N fertilization]; (2) ¹⁵N-urea addition; (3) ¹⁵N maize leaves; (4) ¹⁵N maize leaves + urea; and (5) ¹⁵N-urea + maize straw. The decomposition of SOM and plant residues was traced by the changes of N and C in the light fraction (density < 1.80 g cm^-3) during the 127 days. Urea fertilization decreased the decomposition of SOM and maize residues, as indicated by remaining N and C in the light fraction compared to soil without urea. The C decomposition was tightly coupled to that of N in the light fraction SOM. In soils with maize residues, both maize- and SOM-derived light fractions decomposed slowly with N fertilization. Soil microbial biomass N content was increased by maize residues but was unaffected by urea addition. Under low soil N_min levels, microbes met their N demand by increasing an acquisition from accelerated decomposition of organic sources. To mine N in the N_min limited soils, soil microbes might have directly taken up more N-containing organics and thus facilitated SOM decomposition. For such an acceleration of SOM decomposition, the presence of N uptake by living plants was especially important, which decreased the N_min in soil and so, increased N limitation for microorganisms. We concluded that N fertilization decreases SOM decomposition and increases the efficiency of C sequestration in soil through higher portion of un-decomposed crop residues.

Introduction

Aboveground crop residues are byproducts of agriculture. One benefit of returning crop residues to soil is C sequestration and soil organic matter (SOM) formation. The higher C/N ratio of crop residues than the soil microbial biomass implies that mineral N (N_min) availability may affect the microbial decomposition of crop residues (Sinsabaugh et al., 2013, Cyle et al., 2016, Zang et al., 2016). N fertilization may impact the efficiency of C sequestration through crop residue incorporation. Numerous studies (Recous et al., 1995, Mary et al., 1996, Henriksen and Breland, 1999, Neff et al., 2002, Potthoff et al., 2005) have suggested that high N_min level stimulates the decomposition of plant residues and SOM. Some studies (Neff et al., 2002, Hobbie, 2005, Hobbie et al., 2012; Kaspari et al., 2008) have suggested that the effect of the N_min level on plant residues and SOM decomposition is variable, depending on N content in residues and soil, abundances of other nutrients, organic compound's composition, N leaching and microbial community structure. N fertilization reduces microbial biomass in many ecosystems (Treseder, 2008) and decreases soil CO₂ emissions (Treseder, 2008, Janssens et al., 2010, Spohn et al., 2016, Zang et al., 2016). The decrease in the N_min content changes the decomposer community and accelerates SOM mineralization, resulting in reduced SOM accumulation (Fontaine and Barot, 2005).

The effects of soil N_min on SOM and plant residue decomposition may be biased by the study approach. Current studies on the effects of N_min level on SOM and plant residue decomposition have two limitations: (1) the most of these studies have been conducted in short-term incubation experiments, with mineralization dynamics deduced from CO₂ efflux and N_min changes. In the short-term, the releases of CO₂ and N_min reflect the decomposition kinetics of readily labile compounds (Gunina and Kuzyakov, 2015, Cyle et al., 2016), and thus poorly represent the complex components, which dominate plant residues and SOM. (2) In all incubation studies, the decomposition proceeds in soils without plant growth. In terrestrial ecosystems, microbial decomposition and plant nutrient uptake take place simultaneously, with yielded N_min being removed continuously from decomposition sites.

The present paper quantified the effects of soil N_min availability on the decomposition of SOM and pant residues in the presence of root N uptake. We hypothesized that N fertilization would decrease SOM and plant residue decomposition in soils with growing plants. Despite the fact that soil microbes preferentially use N_min, N_min deficiency leads to organic N uptake by microbes (Hobbie, 2005, Geisseler et al., 2009, Geisseler et al., 2010, Geisseler et al., 2012), which would facilitate N mineralization of SOM and plant residues. The increased N mineralization under low N_min would be linked with increasing organic C mineralization (Jonasson et al., 1999, Manzoni et al., 2010). We investigated the dynamics of various soil N pools, N uptake by wheat (Triticum aestivum L.) and traced the fate of N from applied plant residues and urea fertilizer by using ¹⁵N as a tracer. The decomposition of SOM and plant residues was traced by the changes in light fraction N (LFN) and organic C (LFOC) during 127 days.