Speaker
Description
Introduction
Soil fertility management differs significantly in organic vs. conventional farming systems, as it ideally relies on closing nutrient cycles rather than on the use of external mineral fertilisers’ inputs. While nitrogen (N) fertility is relatively easy to manage through BNF, phosphorus (P) management is more challenging (Möller et al., 2018). Therefore, investigating soil P nutrient flows and status is key to assessing the sustainability of fertilisation practices on organic farms. A number of studies have already attempted to investigate the relationship between farm-gate P budgets and farm types or management practices, with contrasting results (Reimer et al. 2020). However, there is still a knowledge gap in the factors that drive the soil P budgets of organic fields across management practices and farm types.
Methods
We analyse 5-year P budgets for 179 certified organic plots distributed over a wide range of geographical zones in France. The plots were selected to cover a range of farm types, conversion dates, pedoclimatic conditions, as well as cropping practices, in order to identify possible relationships between territorial socio-economic characteristics of farms, P management practices and soil P budgets. Semi-directive interviews were conducted to collect data for the calculation of soil P budgets. Soil budgets (Watson et al., 2002) were calculated for each surveyed plot. Soil P availability was estimated using the Olsen method. The data were analysed using a Linear Mixed Model. As few relationships were identified in the whole dataset, a Factor Analysis of Mixed Data was performed in order to identify associations between management, socio-economic and territorial variables and to analyse the similarity between the plots studied. Subsequently, Hierarchical Clustering on Principal Components was used to identify clusters of similar plots.
Results & Discussion
The data showed a wide range of socio-economic contexts, farm production types and P management strategies. Mean annual soil P budgets ranged from -32.3 to 50.3 kg.P ha-1 y-1. Over the study period, only 7% of the plots had a consistently positive soil P budget, while 30% of plots repeatedly had a negative soil P budget and the remaining 64% had either positive or negative budgets. Soil P budget were mostly explained by the cumulative P inputs, followed by P management, the frequency of N-fixing crops, and frequency of fertilisation. If the plot management was characterized by high P inputs, a high proportion of exogenous P and a low frequency of N-fixing crops, the average P budget tended to be positive. On the contrary, we found a significant negative relationship between the frequency of N-fixing crops in rotations and the P budgets, thus raising concerns about the long-term P fertility of organic farms that rely heavily on BNF. The cluster analyses revealed contrasting situations regarding P management practices and thus soil P budgets. Interestingly, the absence of livestock on the farms led to divergent situations with, on the one hand, cropland plots that were not fertilized at all and, on the other hand, plots that received important quantities of exogenous P inputs. Conversely, plots located on livestock farms tended to have more balanced soil P budgets. In conclusion, a strategic management of P resources and fertilisation is necessary to avoid soil P imbalances that may be difficult to reverse in the medium to long term. Our analysis also shows that the presence of livestock is often, although not always, correlated with a balanced management of soil P status. Thus, the reintegration of crops and livestock in organic systems has a strategic importance also for the P management, in addition to other agronomic benefits.
References
Möller, K., Oberson, A., Bünemann, E.K., Cooper, J., Friedel, J.K., Glæsner, N., Hörtenhuber, S., Løes, A.-K., Mäder, P., Meyer, G., Müller, T., Symanczik, S., Weissengruber, L., Wollmann, I., Magid, J., 2018. Chapter Four - Improved Phosphorus Recycling in Organic Farming: Navigating Between Constraints, in: Sparks, D.L. (Ed.), Advances in Agronomy. Academic Press, pp. 159–237.
Reimer, M., Möller, K., Hartmann, T.E., 2020. Meta-analysis of nutrient budgets in organic farms across Europe. Org. Agr. 10, 65–77.
Watson, C.A., Bengtsson, H., Ebbesvik, M., Lùes, A.-K., Myrbeck, A., Salomon, E., Schroder, J., Stockdale, E.A., 2002. A review of farm-scale nutrient budgets for organic farms as a tool for management of soil fertility. Soil Use and Management 18, 264–273.
| Keywords | Phosphorus management; Organic farming; Soil fertility; |
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