Decreased summer drought affects plant productivity and soil carbon dynamics in a Mediterranean woodland

Abstract

Abstract. Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in extreme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on aboveground net primary production (ANPP) and soil C dynamics, specifically plant-derived C input to soil and soil respiration (SR). Experimental plots were exposed to either a 20 % reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10 % v/v. Treatments were compared to control plots which received ambient precipitation. Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130 %, 26 %, 58 % and 220 %, respectively, as compared to the control. In contrast, the 20 % reduction in throughfall (equivalent to 10 % reduction in precipitation) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or SR. We conclude that minor changes (around 10 % reduction) in precipitation amount are not likely to sig-

Soil C:N stoichiometry controls carbon sink partitioning between above-ground tree biomass and soil organic matter in high fertility forests

“…, and P content of 0.5 g kg -1 (Cotrufo et al 2011). At this site, plant cover is approximately 75% C4 grasses, and for brevity we call henceforth this soil as "C4 soil".…”

“…1 -General characteristics for the six forest sites used in this study. ( Net root-derived C input to soil Net-Croot was quantified using the in-growth core isotope technique, following Cotrufo et al (2011). A soil depleted in 13 C (δ 13 C = -17.22‰) was collected from the USDA-ARS Central Plains Experimental Range located in NE Colorado, USA (40° 49′ N, 104°4 6′ W).…”

“…Prior to C analyses, soil samples were treated with HCl to eliminate carbonates (Harris et al 2001). The measured δ 13 C values were used to calculate the proportion of new C (fnew, i.e., the Net-Croot), by using a mass balance equation (Del Galdo et al 2003, Cotrufo et al 2011…”

“…Estimates of Net-Croot using this method (Cotrufo et al 2011) rely on the assumptions that: (1) root inputs are the same inside and outside the in-growth bags and are independent of the C4 soil properties; and (2) that there is no isotopic fractionation during the decomposition of the native SOM or formation of the new SOM from the root tissues. New studies applying this method should test those assumptions, since some fractionation could occur (Hobbie et al 2004).…”

“…When natural isotope abundances do not allow the use of this approach, distinct C isotope signatures in the soil organic C (SOC) pool and plant-derived organic matter can be obtained in manipulation experiments, by growing C3 plants (δ 13 C of approximately -27‰) in soil with organic matter derived from C4 plants (δ 13 C of approximately -12‰) or vice versa. This approach has been successfully applied in pot (Ineson et al 1995, Vicca et al 2010) and field studies (Hoosbeek et al 2004, Cotrufo et al 2011 and was used in this investigation.Net-Croot, combined with aboveground inputs to the soil (litter and dead wood), also provides interesting information about soil C dynamics. For soils at steady-state (ΔCsoil=0), the sum of Net-Croot and aboveground inputs is the amount of C that replaces SOC decomposition, thus becoming a measure for SOC turnover.…”