Anthropogenic disturbance of natural forest vegetation on calcareous soils alters soil organic matter composition and natural abundance of 13C and 15N in density fractions

Abstract

In the last century, many calcareous soils in Castilla León (northwestern Spain) have been transformed from natural Quercus ilex forest to cropped land. Reforestation with Pinus halepensis has been taking place during the past 40 years. In order to obtain a better understanding of how these disturbances affect ecosystem functioning, we studied the quantity and quality of soil organic matter (SOM) in natural forest ecosystems, cropland and Pinus plantations. Density fractionation combined with ultrasonic dispersion enables separation and study of SOM fractions: free organic matter (OM), OM occluded into aggregates and OM stabilized in organo-mineral complexes, considered on the basis of the type of physical protection provided. We separated SOM density fractions and determined the concentrations of C and N, C/N ratios and the natural isotopic abundance (d 13 C and d 15 N values). Transformation of Quercus forest to cropland resulted in major losses of SOC and N, as expected. However, subsequent reforestation with Pinus resulted in good recovery of the original SOC and soil N pools. This indicates the potential for enhanced C storage in agricultural soils by their reversion to a forested state. Study of the density fractions and their 13 C and 15 N signatures enabled better understanding of the high stability of OM in calcareous soils, and analysis of d 13 C variations throughout the profile also enabled identification of past C3/C4 vegetation change. Despite the different OC contents of soils under different land use, OM stabilization mechanisms were not significantly different. In calcareous soils, accumulation of SOC and N is mainly due to organo-mineral associations, resulting in physicochemical stabilization against further decomposition.

Long‐term Effects of Pine Plantations on Soil Quality in Southern Spain

“…No differences in soil N were detected between pine plantations and their paired unplanted areas and the native forests. This contrasts with many other studies that confirm that N follows the same recovery pattern as SOC after agricultural abandonment and/or afforestation (Dunjó et al, ; Duguy et al, ; Llorente et al, ). Conversely, other studies have demonstrated that afforestation did not ameliorate the soil N with regard to other land uses such as croplands or abandoned fields (Romanyà et al, ) so that most of this element in forest systems is found in the tree biomass (Rosenqvist et al, ).…”

“…Ruíz‐Navarro et al () showed that the influence of afforested pines (planted in terraces) on soil properties was minimal after 30 years and only a thin organic layer was found on the ground. On the contrary, authors such as Dunjó et al (); Llorente et al () or Fernández‐Ondoño et al () found improvements in the physical and chemical soil properties (the latest at catchment scale), and other works have shown no effects or weak positive effects on soil‐C inputs (Castillo et al, ; Nosetto et al, ).…”

“…Also the C : N ratio significantly increased throughout the soil profile. It seems to be assumed that the greatest changes in soil properties occur in the upper part of the soil profile during the restoration process of degraded lands, especially in the uppermost 5 cm (Romanyà et al, 2000;Llorente et al, 2010a;Fernández-Ondoño et al, 2010;Li et al, 2012). However, long-term studies on the effects of conifer plantations regarding SOC and N conducted on former arable land under a temperate climate have revealed differences reaching 10 cm in depth (Rosenqvist et al, 2010).…”

Changes in soil C, N and δ15N along three forest–pasture chronosequences in New Zealand

“…4a), was consistent with other studies (Piccolo et al 1994a;Lemma and Olsson 2006;Llorente et al 2010), and was presumably because processes affecting d 15 N (e.g. volatilisation and nitrification) were higher in the surface horizon (Piccolo et al 1994a).…”

“…N along the chronosequences in this study were similar to those reported when forests have been converted to cropland (Lemenih et al 2005;Lemma and Olsson 2006;Awiti et al 2008;Llorente et al 2010). In those studies the increase in d N to an increase in soil N that was largely derived via N 2 fixation, and hence had a delta value of~0‰.…”