Carbon sequestration rates in organic layers of soils under the Grey poplar (Populus x canescens) stands impacted by heavy metal pollution. Agnieszka MEDYŃSKA-JURASZEK a, Leszek KUCHAR b a Institute of Soil Science and Environmental Protection, b Department of Mathematics, Grunwaldzka 53, Wroclaw, Poland.

Forest role in carbon sequestration Forest ecosystems play a particularly important role in the global carbon budget, because almost 46% of terrestrial organic carbon is stored in tree biomass and forest soils From all of the forests ecosystem elements, forest soil plays the major role as a carbon reservoir, containing twice as much carbon as forest vegetation. Tree biomass Gt C Forest soil Gt C (Puhe and Ulrich 2001). Much pay attention in carbon stocks predicting should be payed to forest floor as the most dynamic part of the soil organic carbon with the highest carbon sequestration potential.

Factors influencing carbon sequestration historical land-use climate change (temperature and precipitation) stand structure (tree species, light interception) age of the tree stand avability of nutrients CO 2 fertilization nitrogen deposition forest use/managment (clear cuttings, thinning, changes in species composition, fire suppression) other human activities eg. SO 2, NO x and heavy metal input from industry

Potential contribution of Polish forests in carbon sequestration Fig. 1 Afforestation in Europe. Source: Report „Forests in Poland 2011”

Potential contribution of Polish forests in carbon sequestration Poland is one of the European countries with the largest forest areas (29.2%) coniferous forest habitats predominate, accounting for 52.1% of the total forest area and pine is a dominate specie the main task in polish afforestation programme is to increase the forest cover to 30% by 2020 and to 33% by 2050

Potential contribution of Polish forests in carbon sequestration according to the estimates based on the available data on timber resources in Poland, the forest biomass contains more than 968 million tonnes of carbon of which about 80% is accumulated in the aboveground biomass. the amount of CO 2 absorbed every year by forests (including soil) is estimated at 52.3 million tonnes, which roughly translates to 14.3 million tonnes of carbon.

Forest soil carbon pools are characterized by slow accumulation rate but fast losses, which makes them quite sensitive for natural and anthropogenic disturbances, and consequently recovery from this disturbance can last very long time.

One of the main disturbances present in industrial areas where afforestation is made for protective purpose is emission of airborne pollutants. This substances maybe harmful for tree plants, inhibiting their growth and metabolism functions but also influencing carbon cycling in whole ecosystem.

Aim of the study The aim of the study was to explore how limit values for decomposition, and thus the litter’s capacity to sequester C, vary between three contrasting sites and to highlight the relationship of the limit value with litter heavy metals contamination.

Study area investigation was carried out in 30 year old poplar stands planted on former arabel soils as a impact zone of the copper smelter copper smelter Legnica is a serious local source of heavy metal pollution (Cu, Pb, Zn, Cd, Hg, As) three humus profiles were sampled at increasing distance from the smelter, downwind all humus profiles were developed on the same soils Cutanic Luvisols.

3 HML (2.1 km) 2 HML (1.5km) 1 HML (0.5km) 11 Localization of the study sites POLAND Warsaw Legnica

Site characterization Site Sanitary conditions Biological activity Humus type Thickness [cm] 1 HML Poor, inhibitated growth and fungal infections Low, inhibitated decomposition rate Mor6 2 HML Good, normal tree growth, no infections Intense, high decomposition rate,earthworm occurance Fresh mull2 3 HML Good, normal tree growth, no infections Intense, high decomposition rate, earthworm occurance Fresh mull2

Chemical properties of humus layers SitepHCu mg·kg -1 Zn mg·kg -1 Pb mg·kg -1 1 HML HML HML

C sequestration in soil organic matter (SOM) is unknown and hard to measure. The present C sink in growing trees can be calculated based on the forest growth and harvest. The long term C sink in forest soil is more complicated to estimate because of a long-term buildup of soil organic matter (SOM) and several methods of C sequestration rate calculation are described in literature.

Limit value model Method was first describe by Berg et al. (2001) and combines quantitative litter fall with data on the remaining recalcitrant fraction of foliar litter, namely that part that becomes soil organic matter (SOM). Fig.2 Asymptotic model for estimating limit values for plant litter decomposition. Redrawn from Berg and Meentemeyer (2002)

What for „limit values” method can be use? Such an estimated limit value may not represent an absolute cessation of decomposition but makes it possible to quantify the remaining, recalcitrant mass in the very late stages of decomposition as well as to quantify SOM buildup.

Factors influencing „limit value” climate microbial activity tree species and chemical composition of litter throughfall initial N concentration in fresh litter initial concentrations of other nutrients (mainly Mn, Ca and lignin) the influence of heavy metals have not been studied previously, but it’s inhibiting effect on organic matter decomposition and increasing buildup of soil organic matter is a well known problem.

Field studies Litter thoughfall traps Litter sampling with steel cylinder

Field studies Litterbags decomposition experiment

Laboratory studies dry mass of the litter thoughfall and litter (kg·m 2-1 ) mass remaining after 6, 9, 12, 16, 20, 24 and 36 months [%] organic matter content in samples determined with loss on ignition method (combustion for 8 h in 505 °C)

Calculations After validation we put are date in to a model proposed by Berg and Ekbohm (1991),and calculated the limit values for each study site using the equation: m.l. - litter mass loss t -time in days m - the asymptotic level that the accumulated mass loss will ultimately reach k - the decomposition rate at the beginning of the decay

The recalcitrant ‘remaining fraction’ was calculated with equation proposed by Berg (2001): Mr = (100 – limit value)/100·Mlt Mr – mass remaining annually [kg/ha] Mlt – annual mass of the litter throughfall [kg/ha] Carbon sequestration was calculated by multiplying the derived SOM buildup by the C fraction in litter decomposed to the limit value, thus forming SOM.

C sequestration rates Site Litter mass [t·ha -1 per year] C pool in litter [Ct·ha -1 per year] Limit value [%] C input with litter fall [Ct·ha -1 per year] C sequestration rate [Ct·ha -1 per year] 1 HML HML HML

Decomposition rates

Conclusions 1)Limit values calculated for O horizons with different levels of heavy metals differed between contrasting sites 1 HML and 2,3 HML. 2)The lowest limit value was found in O horizons at site 1 HML and there was a significant negative correlation between limit values and heavy metal concentrations. 3)There was negative correlation between C pool of the organic layer and the C sequestration rate. 4)High heavy metal concentration is a limiting factor for litter decomposition causing high accumulation of organic matter and low C sequestration in forest litter. 5)Limit value method is predictive and includes the effect of chemical composition of organic matter, but further investigation are neccesary.

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