Soil properties and not inputs control carbon, nitrogen, phosphorus ratios in cropped soils in the long-term

Abstract

Abstract. Stoichiometric approaches have been applied to understand the relationship between soil organic matter dynamics and biological nutrient transformations. However, very few studies explicitly considered the effects of agricultural management practices on soil C : N : P ratio. The aim of this study was to assess how different input types and rates would affect the C : N : P molar ratios of bulk soil, organic matter and microbial biomass in cropped soils in the long-term. Thus, we analysed the C, N and P inputs and budgets as well as soil properties in three long-term experiments established on different soil types: the Saria soil fertility trial (Burkina Faso), the Wagga Wagga rotation/stubble management/soil preparation trial (Australia), and the DOK cropping system trial (Switzerland). In each of these trials, there was a large range of C, N and P inputs which had a strong impact on element concentrations in soils. However, although C : N : P ratios of the inputs were highly variable, they had only weak effects on soil C : N : P ratios. At Saria, a positive correlation was found between the N : P ratio of inputs and microbial biomass, while no relation was observed between the nutrient ratios of inputs and soil organic matter. At Wagga Wagga, the C : P ratio of inputs was significantly correlated to total soil C : P, N : P and C : N ratios, but had no impact on the elemental composition of microbial biomass. In the DOK trial, a positive correlation was found between the C budget and the C to organic P ratio in soils, while the nutrient ratios of inputs were not related to those in the microbial biomass. We argue that these responses are due to differences in soil properties among sites. At Saria, the soil is dominated by quartz and some kaolinite, has a coarse texture, a fragile structure and a low nutrient content. Thus, microorganisms feed on inputs (plant residues, manure). In contrast, the soil at Wagga Wagga contains illite and haematite, is richer in clay and nutrients and has a stable structure. Thus, organic matter is protected from mineralization and can therefore accumulate, allowing microorganisms to feed on soil nutrients and to keep a constant C : N : P ratio. The DOK soil represents an intermediate situation, with high nutrient concentrations, but a rather fragile soil structure, where organic matter does not accumulate. We conclude that the study of C, N, and P ratios is important to understand the functioning of cropped soils in the long-term, but that it must be coupled with a precise assessment of element inputs and budgets in the system and a good understanding of the ability of soils to stabilize C, N and P compounds.

Nitrogen and phosphorus uptake from isotope‐labeled fertilizers by sorghum and soil microorganisms

“…This is in accordance with Frossard et al. (2016), who found that microorganisms in a similar nutrient depleted soil from a field experiment in the same region are highly dependent on nutrient inputs. However, microbial P in the treatment mineral 1N1P was significantly lower than after addition of labeled young cowpea residues, which brought similar amounts of N and P to the soil, but combined with C. This agrees with Traoré et al.…”

“…The SMB took up N and P to meet its internal demand at the level of its biomass which is firstly limited by C. This agrees with Frossard et al. (2016), who found that N/P ratios in the SMB were strongly and positively correlated with the N/P ratios of the nutrient inputs in different fertilizer treatments of a long‐term trial on a ferric Acrisol from the same location as the studied soil. This also suggests N and P limitations for the SMB in this soil (Traoré et al., 2016).…”

“…The low N/P ratio of SMB in the manure treatment is likely due to the high availability and uptake of P compared with N added by this manure. This also reflects a plasticity in the microbial N/P ratios depending on the availability of the nutrients added by fertilizers and organic inputs in nutrient‐depleted soils (Frossard et al., 2016). The N/P ratios of the SMB depend on the measured values of microbial N and P. These values were not corrected for incomplete recovery from the soil during extraction, because the conversion factors were not determined for the studied soil.…”

Impacts of preferential flow and agroecosystem management on subsurface particulate phosphorus loadings in tile‐drained landscapes

“…We generated a linear regression between daily tile TP − DRP and TN − DIN loading for samples measured using alkaline persulfate digestion (before 2015) and alkaline/acid persulfate digestion (after 2015). Next, we compared the results with typical P/N ratios for organic matter in agroecosystems, which have been found to range from 0.034 to 0.083 (Frossard et al., 2016). These ratios for organic matter have shown limited variability across comparable landcovers (Cleveland & Liptzin, 2007).…”

“…The difference between TP and DRP (TP − DRP) reflects the sum of inorganic PP, organic PP, and dissolved unreactive P (Macrae et al., 2019), whereas the difference between TN and dissolved inorganic N (DIN) (TN − DIN) reflects organic (particulate and dissolved) N species (Patton & Kryskalla, 2003). Organic compounds have predictable P/N ratios in soil organic matter that are significantly less than P/N ratios of the bulk soil pool in row‐cropping systems, which stems from accrual of inorganic P in soils (Cleveland & Liptzin, 2007; Frossard et al., 2016). Comparing ratios of TP − DRP with TN − DIN can aid in informing forms of P delivered to tile drains and provide insight into where PP is mobilized in the soil profile.…”