Abstract
Nitrogen is an essential component in forest ecosystem nutrient cycling. Nitrogen fractions, such as dissolved nitrogen, ammonium, nitrate, and microbial biomass nitrogen, are sensitive indicators of soil nitrogen pools which affect soil fertility and nutrient cycling. However, the responses of nitrogen fractions in forest soils to organic mulching are less well understood. The rhizosphere is an important micro-region that must be considered to better understand element cycling between plants and the soil. A field investigation was carried out on the effect of mulching soil in a 15-year-old Ligustrum lucidum urban plantation. Changes in total nitrogen and nitrogen fractions in rhizosphere and bulk soil in the topsoil (upper 20 cm) and in the subsoil (20–40 cm) were evaluated following different levels of mulching, in addition to nitrogen contents in fine roots, leaves, and organic mulch. The relationships between nitrogen fractions and other measured variables were analysed. Organic mulching had no significant effect on most nitrogen fractions except for the rhizosphere microbial biomass nitrogen (MBN), and the thinnest (5 cm) mulching layer showed greater effects than other treatments. Rhizosphere MBN was more sensitive to mulching compared to bulk soil, and was more affected by soil environmental changes. Season and soil depth had more pronounced effects on nitrogen fractions than mulching. Total nitrogen and dissolved nitrogen were correlated to soil phosphorus, whereas other nitrogen fractions were strongly affected by soil physical properties (temperature, water content, bulk density). Mulching also decreased leaf nitrogen content, which was more related to soil nitrogen fractions (except for MBN) than nitrogen contents in either fine roots or organic mulch. Frequent applications of small quantities of organic mulch contribute to nitrogen transformation and utilization in urban forests.
Highlights
Organic mulching plays an important role in urban soil nutrient conservation and restoration
Most studies that have evaluated the effects of organic mulching on the soil environment have tended to focus on agricultural soils, whereas studies on these effects in forest ecosystems, which may be characterised by markedly contrasting dynamics, are rare
Our results suggest that microbial biomass nitrogen is more sensitive than other nitrogen fractions to organic mulching, and MBN in rhizosphere was more sensitive than in bulk soil, and strongly affected by changes in the soil environmental
Summary
Organic mulching plays an important role in urban soil nutrient conservation and restoration. In addition to its physical effects such as water conservation and temperature regulation, organic mulching contributes carbon (C) and nutrients to the soil, with the interaction of multiple factors resulting in complex mechanisms responsible for changes in the soil environment (Kader et al 2017). Nitrogen (N) plays a prominent role in controlling the primary production of plants in terrestrial ecosystems (Guignard et al 2017). The accumulation of nitrate as a consequence of continuous N addition tends to cause soil acidification (Lucas et al 2011), which can have inhibitory effects on microbial activities related to plant growth and N fixation (Vitousek et al 1997). The underlying mechanisms of N changes are complex and remain to be confirmed
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