Testing soil phosphorus in a depleting P scenario: an accelerated soil mining experiment

Abstract

Summary The reduced use of phosphorus (P) fertilizer in fertile soil has reverted the P balance to negative in some regions. It is unclear how long current soil P stocks will ensure adequate P supply to crops. In addition, it is unknown if current soil tests for available P describe bioavailable P adequately in soil where P is becoming depleted. We set up an accelerated soil P mining test to address these questions. Perennial ryegrass (Lolium perenne, Melpetra tetra) was grown for 2 years in a greenhouse on 5‐cm‐deep soil layers of eight contrasting soils with periodic grass clipping. Each soil was split into four fertilizer treatments (i.e. no P (–P) and adequate P (+P)) and two nitrogen levels, the latter to alter the rate of P uptake. The long‐term P mining induced P‐related yield losses in seven of the 16 soil treatments. The cumulative uptake of shoot P at which yield loss started to exceed 10% (–P versus +P) varied over a small range of 37–74 mg P kg−1 soil among the soils. This critical cumulative P uptake (CCP) was related to the soil P content prior to mining measured by five soil P tests (ammonium oxalate, ammonium lactate (AL), Olsen P, 0.01 m CaCl2 and the diffusive gradient in thin film technique (DGT)); the largest R2 values were observed for P‐AL (R2 = 0.72) and P‐DGT (R2 = 0.73). However, none of the tests was diagnostic for yield loss during the depletion period. Increased N supply accelerated growth and rates of P uptake and decreased the CCP by a factor of 1.7 on average, illustrating the effect of the rate of biomass production. The CCP values obtained in the treatment with reduced N fertilizer application are likely to be the most relevant for the field and suggest that current stocks allow adequate P supply for arable crops for 3–8 years under zero P application (0–23 cm) in soils similar to those tested. The lack of a successful diagnosis for P deficiency during this depletion experiment calls for further calibration of soil tests for available P in the field. Highlights The availability of legacy P in well‐fertilized soil was evaluated with a P mining pot trial 10% loss of crop growth occurred when soil P was depleted by 37–74 mg P kg−1 soil Accelerated plant growth with increased N supply decreased total P uptake beyond which P deficiency occurs In a depletion scenario, current soil P tests are not diagnostic but they can be used for prediction

The phosphate desorption rate in soil limits phosphorus bioavailability to crops

“…Phosphorus desorption curves for soil #8 of the collection described elsewhere (Nawara et al, 2018) that was tested after (final) a 2‐year P mining pot trial in the glasshouse and on the corresponding initial sample. The final difference in desorption is denoted as ΔP des and equals 53 mg kg −1 in this example.…”

“…This was done for three soils, each of these before and after mining; that is, soil 1 (light loam), soil 5 (sand) and soil 8 (loam), the Ɵ v was set to 0.4, 0.3 and 0.45, respectively (estimated for water saturated soils), and the associated f = 0.4, 0.3 and 0.45 (Barraclough & Tinker, 1981). The predicted P fluxes in these six soils ranged from 0.49 to 1.18 μg P cm −2 day −1 , whereas corresponding observed fluxes to the DGT ranged from 0.22 to 1.60 μg P cm −2 day −1 (Nawara et al, 2018); the predicted values correlated well with observed values (R 2 = 0.61, predicted/observed ratio 0.7–2.2, mean 1.4). The overestimation is likely to be related to not including the 0.94‐mm diffusion layer as discussed above.…”

“…A 1D diffusion test on unsaturated soil mimicking the model has not been carried out for the soils here. However, all soils were tested with a DGT; these data were obtained using 24–48 hr DGT deployment times on saturated soils and data have been reported (Nawara et al, 2018). DGT is a sink‐based sampling device.…”

“…This model predicts P uptake by the grass of the 2‐year P mining trial under the zero‐P treatment (Nawara et al, 2018), thereby using soil P supply kinetics from the desorption experiment and plant parameters from the treatment in which P was not limiting. The model is a modification of the classic mechanistic Barber‐Cushman model for nutrient uptake (Barber, 1995) in which nutrient supply by mass flow and diffusion is combined with root growth and ion uptake kinetics at the root surface.…”

“…In the latter scenario, shoot P concentrations will decrease, which ultimately affects the plant growth. The plant demand is described in the model as the optimal (potential) growth multiplied with its optimal P content, which was set to 5 mg P g −1 dry weight based on the maximal shoot P concentrations observed in the –P treatment during the P‐depletion experiment (Nawara et al, 2018). The optimal growth was based on the observed values of the +P treatment, that growth varied markedly over time due to seasonal changes in light and temperature in the glasshouse.…”