Effects of land use type and tree age on some soil properties and organic carbon and total nitrogen stock capacity

Carbon dioxide is a dominant greenhouse gas. Increased atmospheric CO2 is attributable mostly to fossil fuel combustion (about 80–85%) and deforestation worldwide. Atmospheric carbon is estimated to be increasing by approximately 2600 million metrictons annually. Nitrogen (N) deposition has also increased more than tenfold since the last 150 years due to the intensification of human activities such as production and application of nitrogenous fertilizer, and fossil fuel combustion. The increasing N availability can impact C and N cycling in terrestrial ecosystems. Forest trees act as a sink for CO2 by fixing carbon during photosynthesis and storing excess carbon as biomass. The net long-term CO2 source/sink dynamics of forests change through time as trees grow, die, and decay. In addition, human influences on forests (e.g. management, land use change) can further affect CO2 source/sink dynamics of forests through such factors as fossil fuel emissions and harvesting/utilization of biomass. For those reasons, it is neccerassary to gain more knowledge on the role of forest ecosystems especially forest soil carbon and total nitrogen pools for mitigation of greenhouse gases and the factors infulencing soil organic carbon and total nitrogen stock capacities. Main aim of this present study was to investigate the effects of tree age and land-use change on soil properties, soil organic carbon and total nitrogen contents and stock capacities, forest floor litter amount, chemical constituents and litter decomposition dynamics using the most common tree species, fir and Scots pine in Kastamonu Region. The study was carried out at two areas (Inebolu and Ilgaz). Under similar microclimate conditions, in Inebolu area, soil samples and forest floor material were collected from 4 different ages of fir (38, 60, 90 and 100 year-old), 2 different ages of Scots pine (18 and 30 year-old) and one agriculture site, while in Ilgaz area, soil samples were collected from 6 different ages of fir (57, 66, 183, 250, 283 and 306 year-old) and one pasture site. The soil samples were taken from 5 different soil depths (0-5, 5-10, 10-15, 15-20 and 20-30 cm) using soil cores and analyzed for soil pH, texture, soil nutrients, soil bulk density, organic carbon and nitrogen contents and soil organic carbon (SOC) and total nitrogen (TN) stock capacities. In addition, in Inebolu site, the effects of different tree ages on litter decomposition rates of fir and Scots pine were studied using a litter bag method. Fir litters (38, 60, 90 and 100 year olds) and Scots pine litters (18 and 30 year olds) were seperately collected and put in the litter bags. The litter bags were then placed on the corresponding sites and sampled every 6 month for 2 years. The results showed that the soil properties, soil nutrients, SOC and TN contents, initial litter biochemical composition varied between the tree ages and soil depths, but there were significant differences in SOC-and TN-stock capacities and litter decomposition rates among tree ages. When 0-30 cm depth was considered, in Inebolu site, the lowest SOC stock capacity (86,60 t C ha-1) was seen under the 30 yr-old Scots pine trees, followed by the agriculture site (93,70 t C ha-1), 90 yr-old fir trees (94,10 t C ha-1), 18 yr-old Scots pine trees (115,0 t C ha-1), 60 yr-old fir trees (120,0 t C ha-1), 38 yr-old fir trees (146,10 t C ha-1) and the 100 yr-old fir trees (160,4 t C ha-1). Nitrogen stock capacity was the highest for 100 yr-old fir trees and 18 yr-old Scots pine trees (8,97 t N ha-1 and 7,86 t N ha-1 respectively), while it was lowest for 30 yr-old Scots pine and 60 yr-old fir trees (5,74 t N ha-1 and 7,04 t N ha-1 respectively). In Ilgaz site, the 66 yr-old fir trees had the lowest soil carbon stock capacity (96,46 t C ha-1), followed by pasture land (113,70 t C ha-1), 183 yr-old (145,30 t C ha-1), 250 yr-old (145,40 t C ha-1), 283 yr-old (146,30 t C ha-1), 306 yr-old (160,50 t C ha-1) and 57 yr-old (166,70 t C ha-1). Nitrogen stock capacity was highest for pasture soils and 283 yr-old fir trees (9,50 t N ha-1 and 9,07 t N ha-1 respectively), while it was lowest for 66 and 250 yr-old fir trees (6,04 t N ha-1 and 7,29 t N ha-1 respectively). As for the litter decomposition experiments, after 24 months, 60 year-old fir litters showed the highest mass losses (80,2%), followed by 100 year-old fir litters (75,8%), 90 year-old fir litters (69,4%) and 38 year-old fir litters (67,7%). 18 year-old Scots pine litter decomposed faster than the 30 year old litters (76,7%). Initial carbon concentration explained most of the variation in decomposition rates between the litter ages (r2= 0.723 after 6 months, r2= 0.514 after 12 months, r2= 0.946 after 18 months and r2= 0.895 after 24 months). In conclusion, the results from this present study illustrate the important point that tree ages and land use changes can strongly influences soil properties, soil organic carbon and total nitrogen contents and stock capacities, quality and litter decomposition dynamics. Therefore, differences in tree age and land use changes should be considered in future studies. However, more detailed studies are needed using different tree species to reach a final conclusion.