Effects of waterlogging on the soil structure of some Italian soils in relation to the GAEC cross-compliance standard Maintenance of farm channel networks and field convexity

The aim of this work is to assess the effectiveness of the cross-compliance standard Maintenance of farm channel networks and field convexity with respect to the environmental objective to maintain soil structure through appropriate measures indicated in Annex IV of REG. EC 1782/03. This GAEC standard concerns arable land and obliges the farmer to maintain the farm channel network and the convex shape of fields with a view to the management and preservation of temporary drainage ditches and permanent channels along the edges of the fields, in order to ensure their efficacy and function in draining away water. Experiments have confirmed that maintaining the farm channel networks and giving fields a convex shape has a positive effect in predisposing the soil to ideal conditions for the preservation of its structure. In particular, we have used the data of a newly conducted laboratory study on the effects of the duration of waterlogging on the stability of soil structure, as well as the results of several studies carried out in the past with the following goals: i) assessment of the effects of soil moisture various levels on the stability of the structure; ii) assessment of the effects of several wetting and drying cycles on aggregate stability; iii) determination of aggregate stability under the influence of freezing and thawing under different soil moisture conditions; iv) determination of aggregate stability with an increasing number of freeze/thaw cycles (0, 1, 3, 5, 11 cycles) at field capacity moisture. The tests during which the soil was subjected to freezing and thawing were carried out taking into account the fact that during winter waterlogged soil may freeze, leading to a structure breakdown, due to the expansion of water into the pores during the freezing phase. In general, the results showed that in soils that are sandy and rich in rock fragments, rapid moisture penetration leads to significant disaggregation phenomena compared with soils with higher silt and clay contents. The results also showed that the least extensive disaggregation of the structure occurred with three wetting/drying cycles. This behaviour was more obvious in soils with a greater silt and clay content compared with sandy loam soils. After the third wetting/drying cycle, disaggregation increased with the cycles number increasing. In particular for moisture contents of 50%, this test confirms that maintaining the farm channels efficacy has a beneficial effect on soil structure, as good draining of water, which is ensured by well-functioning ditches, prevents waterlogging and fights the dispersion of clay and silt (when the soil is dry, these two components form a surface crust that disturbs the water and gas exchange as well as plant growth). Disaggregation due to freezing and thawing is linked very closely to the matrix potential (and hence to the moisture) of the aggregates before treatment. In general it can be said that a water content between saturation and field capacity predisposes the soils to a much higher level of disaggregation compared to soil that is air-dry or at the wilting point. The tests that examined the effect of an increasing number of freeze/thaw cycles showed a logarithmic degradation of structural stability in relation to the number of cycles. However, most of the disaggregation occurred after three freeze/thaw cycles. This experiment showed that only few freeze/thaw cycles are necessary to cause a significant structural degradation of soils with high water content and with a texture ranging from loamy to clayey. The results of the studies allow us to deduce that the Standard is efficient in contributing to the preservation of the soil structure. In fact, maintaining the water drainage network in efficient conditions and giving the fields a convex shape are elements that contribute to the removal of excess water, which results in a decrease of both of the possibility of the dispersal of silt+clay microaggregates and of the risk of structural damages due to freezing and thawing.