INTRODUCTION

Most olive groves are managed by tillage in the Madrid Region (>71%), as in other Spanish regions (MAPAMA, 2019). This practice leads to soil erosion with an important loss of organic carbon and nutrients (^{Gómez et al., 2009}; ^{Bienes et al., 2010}).

Alternative soil managements are crucial to avoid soil and nutrient loss, in order to reach a sustainable agriculture. Permanent or annual cover crops, seeded or spontaneous, and mulching are strategies to protect the soil from erosive processes (FAO & ITPS, 2021; ^{Sastre et al., 2017}). Moreover, carbon sequestration in soils is a key process for adaptation and mitigation of the expected negative impacts of Climate Change (^{Rodríguez Martín et al., 2016}).

MATERIAL AND METHODS

This work was performed mainly in “Comarca de Las Vegas”, a shire at the south-east of Madrid Region with high extensions of woody crops (mainly olive groves). The climate is classified as Continental Mediterranean, with a mean annual temperature of 14.7ºC, accumulated annual precipitation of 370 mm, and a reference evapo-transpiration (ET₀ Penman-Monteith) of 1170 mm.

The first stage of the project was to find farmers who did not use tillage as soil management in their woody crops. More than 30 plots were visited, selecting 15 private rainfed olive groves, and adding two research IMIDRA's farms to the trial (Figure 1): “La Chimenea” and “El Socorro”. These are experimental farms with olive orchads and vineyards, respectively.

Once the soil alternative management plots were selected, a close farm with the same crop and similar soil was chosen. From the 17 plots with soil conservation management 7 were mechanically mowed once or twice per year (mean of 7 years with this management), 3 were yearly chemically mowed (mean of 6 years with this management), and 7 had no soil management due to the scarcity of the spontaneous vegetation growing in these soils (mean of 6 years with this management).

Soils were classified according to the FAO’s map (Centro de Investigaciones Agrarias del CSIC (Ministerio de Economía, Industria y Competitividad) & Consejería de Medio Ambiente y Ordenación del Territorio (Comunidad de Madrid), 1990) as follows: 2 Calcisols, 1 Cambisol, 2 Gypsisols, 5 Leptosols, 6 Luvisols and 1 Regosol.

Figure 1 Plots location in Madrid Region (n=16, each dot is a pair of plots: soil conservation management and tillage). 

In each plot, 3 composite samples were taken in the inter-rows at 4 depths: 0-5, 5-10, 10-20 and 20-30 cm. The samples were sieved at 2 mm, to analyze soil organic carbon (SOC) by wet oxidation method (^{Walkley & Black, 1934}). SOC stock was obtained as follows:

C Stock = conc. × BD × d × (1-S) × 10²

where C Stock is the stock of carbon (Mg ha^-1); conc. is the concentration of carbon (%); BD is the bulk density (Mg m^-3); d is the thickness (m); and S is the stoniness (% of coarse fragments >2 mm).

One and two-way ANOVA tests were used for the groups of managements to establish significant differences between variables (LSD test for differences).

RESULTS AND DISCUSSION

SOC stock at 0-30 cm depth was statistically different for the different soil management (Figure 2). The means were 33.5 [18.7-66.4], 31.6 [23.2-39.8], 27.0 [19.0-32.7] and 28.6 [17.0-44.3] Mg·ha^-1 for Mecanical mowing, Chemical mowing, with No management and Tillage, respectively (minimum and maximum values in brackets). Only under mechanical mowing there were statistical significant differences regarding Tillage. These results of carbon stock are higher than those found by ^{Calvo de Anta et al. (2020}) in soils of woody crops from Madrid (24 Mg SOC·ha^-1); but quite below to those described for soils in woody crops in Spain by ^{Rodríguez Martín et al. (2016}) being 38.09 Mg·ha^-1 of SOC; or those considered by ^{Bateni et al. (2019}) in olive groves in Italy , being 36-71 Mg·ha^-1. This lower content of SOC stock is due to the edaphoclimatic conditions (Calvo de Anta et al., 2020), since this area is under semiarid or close to semiarid climate with thin soils, some of them with high content of gypsum or lime.

Figure 2 Soil Organic Carbon (SOC) stock at 0-30 cm depth under different soil management. Different letter above the column means statistically significant differences according LSD test (p<0.05). 

SOC stock in woody crops with a groundcover managed with a mechanical mowing increases by 20% regarding tillage. It means an increase of 4.9 Mg·ha^-1 in groundcovers managed mechanically mowed for 7 years on average. This result is consistent with previous works of the group (^{Sastre et al., 2018}), where an increase of SOC stock of 1 Mg·ha^-1 year^-1 under the permanent cover crop at 0-10 cm depth was measured. However, these results are far from those from ^{Nieto et al. (2013}) who measured an increase of 4.02 Mg·ha^-1·year^-1 in olive groves with cover crops in southern Spain. This rise in 4.9 Mg·ha^-1 SOC stock could be the limit that this type of soil under this climate could accumulate.

SOC stock variation between the pair of plots (Soil conservation management and Tillage) was calculated in order to evaluate the effect of the soil conservation management classes, for the same edaphoclimatic conditions (Figure 3). The main differences appears between Mechanical mowing in the upper horizons.

Figure 3 Soil Organic Carbon (SOC) stock differences among soil conservation management and tillage in each plot. 

A Two-way ANOVA was performed to determine the effect of the management and depth in the variations in SOC stock under the soil conservation management regarding tillage (Table 1).

Both factors, Soil Treatment and Depth, were significantly different, and there was no interaction between factors. Woody crops managed under Mechanical mowing stores significatively more SOC compared with tillage than the other two managements, that are similar between them. The largest differences in SOC variationappear at 0-5 cm depth, being statistically different than the variations at 10-20 and 20-30 cm depth. This is consequence of SOC stratification caused by the contribution of plant debris in the topsoil. Tillage homogenization is absent, contributing to a layered differentiation.

CONCLUSIONS

SOC stock depends on the soil conservation management applied and on the local edaphoclimatic conditions. Only Mechanically mowing increase SOC stock at 0-30 cm regarding tillage, but there were no differences with Chemical mowing or No treatment. This information should be taken into account in order to be considered among the “greening” measures of the next Common Agricultural Policy. The 'green direct payment' (or 'greening') supports farmers who adopt or maintain farming practices that contribute to EU environmental and climate goals.