P dynamics in rainfed forage maize crop soils under different maize-grassland rotation cycles

In a context of environmental sustainability, fertiliser application to agricultural soils must be optimised through sustainable cultivation practices. Over-fertilisation, the use of soluble mineral forms and/or their application at inappropriate stages of crop growth, has generated serious problems associated with the alteration of biogeochemical cycles (mainly C, N and P), eutrophication of waters, emission of greenhouse gases and depletion of natural resources. For this reason, our research team is conducting a project aimed at developing and evaluating sustainable agricultural soil management practices that reduce dependence on inorganic fertilisers and pesticides, prevent SOM loss and erosion, and contribute to the restoration of soil biodiversity. As part of this project, this study focuses on assessing P dynamics in rainfed forage maize crop soils subjected to different management practices, aiming to evaluate the influence of these practices on the bioavailability of P and its potential leaching into drainage waters.

We selected several maize plots subjected to two distinct management types, each under conventional inorganic/organic fertilisation regime. The first management type follows a typical rotation for maize crops in the area - after maize harvest (September-October) the plot is maintained as grassland for 2-3 years before being cultivated with maize again (3-year rotation). The second management type focuses on analising P dynamics in soils during the early stages of conversion to cultivation. In this case, we choose a site where soil that had been under grassland for at least 100 years was converted to maize; however, in this case the maize-grassland rotation is annual, with the two crops alternating each year (1-year rotation).

Over three consecutive years, samples were taken from the top 10 cm of the soil under the triennial grassland-maize rotation, while for the annual rotation, samples were taken from the top two soil layers (0-10 and 10-20 cm). Soil sampling occurred at various times when the soil was under maize and grassland. P forms were analysed following the sequential fractionation of Hedley et al. (1982). Residual inorganic and total P were analysed through extraction with 0.5 N sulfuric acid before and after calcination (550 ºC, 2 h) of the fractionation residue, estimating the residual organic phosphorus by difference between both. The pseudo-total P content of the soils was determined by ICP-OES after acid digestion with HNO₃ +HCl in a microwave oven (MILESTONE, Italy).

The results were analysed in relation to the type of rotation and the time elapsed since the transformation from grassland to maize cropland. The P content, especially inorganic P, in the soil subjected to the 3-year rotation was significantly higher compared to the soil under the 1-year rotation. This discrepancy reflects the historical overfertilization experienced by the former over an extended period. Moreover, the transition from grassland to maize cultivation resulted in the loss of the typical stratification observed in grassland soils. This was evidenced through the homogenization of P contents across all organic and inorganic forms in the two soil layers investigated.

Hedley M.J., J. Stewart J.W.B., Chauhan B.S. 1982. Soil Sci. Soc. Am. J. 46, 970-976.