Does carbon limitation reduce nitrogen retention in soil?

Abstract

Artificial soils made from waste materials offer an alternative to imported natural top-soils, notably in large-scale groundworks and reclamation projects. Benefits include diversion of waste from landfill and recycling. Nonetheless, there is limited information on the characteristics needed to support plant growth in the long-term, particularly the existence of a sustainable nitrogen reservoir. Therefore we assessed the efficacy of nitrogen cycling and retention within an artificial soil composed of 25 % sand, 10 % clay, 32.5 % composted bark and 32.5 % composted green waste over 52 weeks. Leachate was analysed for nitrogen species and nitrogen concentrations and two of the soil columns had fertiliser added after 26 and 48 weeks.Results show that nitrate concentrations decreased from 6.73 to 0.36 mg N L -1 after 2 weeks, due to poor retention of this anion in soil, and remained low for 6 months, before increasing up to 5.87 mg N L -1 after week 26, in unfertilised soils. The sharp increase in dissolved nitrate was preceded by a decrease of the ratio of dissolved organic carbon over dissolved organic nitrogen in the soil leachate. This finding indicates that the soil had become carbonlimited, leading to mineralisation of organic nitrogen by soil organisms and excretion of nitrogen.We also found that fertilisation of the soil with nitrogen-rich substrate did not alleviate carbon-limitation and nitrogen-loss was greater in fertilised soils, indicating nitrogensaturation. After the onset of carbon-limitation, the dissolved nitrate concentrations in both the fertilised and unfertilised soils were close to exceeding the European Union threshold of concern for nitrate groundwater and river pollution. Thus while the deployment of artificial soils is a viable option for landscaping projects, loss of nitrogen may be environmentally significant and soil management protocols must take account of both the carbon and nitrogen status of the substrate.

Biochar incorporation increased nitrogen and carbon retention in a waste-derived soil

“…The mesocosms were maintained unplanted and covered, to minimise evaporative losses, in a controlled temperature room (15 ºC) for 6 weeks. The temperature was that employed during previous experiments on the soil was within the annual range reported for the region Schofield et al (2018). In that study, irrigation of the soil over 6 weeks reduced NO3 -, DON and DOC concentrations by 99, 36 and 27 %, respectively.…”

“…Under conditions of constant temperature and moisture, microbial diversity within soils is impacted predominantly by soil pH, carbon to nitrogen (C : N) ratio and, to a lesser extent, phosphorus (Dumbrell et al 2010). A previous study of a manufactured soil linked high C : N ratios to carbon limitation in the soils, leading to mineralisation of soil organic nitrogen (Schofield et al 2018). This was evident from a sustained increase in dissolved nitrate concentrations in soil leachate as the nitrogen within the organic molecules was quickly converted to this form (Bingham and Cotrufo 2016).…”

“…This study aimed to evaluate the impact of biochar on the efficacy of nitrogen retention, both organic and inorganic, storage and release within a manufactured soil. The test soil, composed of waste materials, has been deployed to support a variety of plants within natural and artificial environments over a 15-year timescale; however, its success as a growth medium has relied on regular fertiliser applications to supply the required nutrients in plant-available form, and significant losses of carbon and nitrogen were apparent in leachate from soil columns measured over a 12-month period (Schofield et al 2018). The objective of the study was to measure the effect of biochar application to the manufactured soil, at 3 concentrations, on the retention of macronutrients over the experimental period.…”

“…The air-filled porosity was 25 %, measured through assessments of air-filled porosity of the freshly-prepared substrate following the procedure of Bragg and Chambers (1988). Further details on the soil are given in Schofield et al (2018).…”

“…retention of N and C in the soil recipe tested. Each mesocosm was irrigated with 10 mL day -1 (0.84 mL m -2 d -1 ) HPW adjusted to pH 7 (Schofield et al 2018).…”