CARBON AND NITROGEN STOCKS AND C : N RATIOS OF HARRAN PLAIN SOILS

Previous studies have focused on carbon (C) and nitrogen (N) stocks of soils because of increases in atmospheric carbon dioxide (CO2) and terrestrial ecosystems with wide N storages. The goal of this study was to determine C amounts and stocks that are important for global warming, N amounts and stocks and C:N ratios. To this end, 16 series were opened on the Harran Plain and soil samples were taken from 100 cm depth and each horizon. The results showed that total carbon amounts varied between 0.80 and 1.85 kg C m –2 . N amounts were between 0.16 and 0.34 kg N m –2 , C:N ratios were between 4.32:1 and 6.04:1 and bulk density (BD) was 1.23-1.34 Mg m −3 . Carbon and N stocks were determined as 10.53 Tg C and 1.96 Tg N, respectively.


INTRODUCTION
he importance of the biogeochemical cycles of carbon and nitrogen has increased in terrestrial ecosystems throughout the world, because their oxidation into the atmosphere plays an important role in global warming maintenance.Soil organic carbon (SOC) is the largest component of terrestrial carbon, and the amount of carbon that exists as SOC is two to three times greater than the carbon present in live vegetation (Post and Kwon, 2000).Moreover, changes in SOC pools can increase the carbon dioxide (CO 2 ) concentrations in the atmosphere (Smith, 2008).Therefore, understanding soil carbon storage potential and developing effective methods to decrease the atmospheric CO 2 concentration are vitally important (Fu et al., 2010).
Many factors affect the biogeochemical cycle of SOC and therefore also affect SOC stocks and distribution.One of the most important factors is land-use alteration.While converting natural forest areas and meadows to farming areas affects SOC stocks in different ways in different ecosystems and regions (Solomon et al., 2000;Rodriguez-Murillo, 2001;Powers, 2004;Yimer et al., 2007), cultivation and other deformations cause approximately 40 Pg of C loss.Nearly 1.6 Pg C y −1 were released in the 1990s (Smith, 2008).Developing non-cultivated agriculture techniques can decreases SOC pool decrease, which emerged as a result of natural and forest areas decrease (Puget and Lal, 2005;Grandy and Robertson, 2007).
Soil is one of the most important C and N pools and includes approximately 75% OC and 95% N (Schlesinger, 1997).The interaction between SOC and N is affected by the plants that are present, which affects the ecosystem yield and the terrestrial C cycle.Numerical models of C and N cycles include terms for the climate, the atmosphere and land-use alternation (Homann et al., 2000;Kirschbaum, 2000;Pepper et al., 2005).Jenny (1941) synthesised and summarised the interaction between climate and soil humidity.According to her study, when humidity increases, the N ratio in soil increases in the meadows in the central and eastern United States, but the effects on forests are minor.The N ratio in soil also increases when the temperature increases.Sandy soils contain less mineral-organic C and N than loamy soils.The C:N ratio of sandy soils is higher than that of loamy soils.Similar studies were conducted simultaneously in the central United States (Burke et al., 1989;Franzmeier et al., 1985;Sims and Nielsen 1986), as well as in other parts of the country (Conant et al., 1998;Grigal and Ohmann, 1992;Homann et al., 2004) and in other countries of the world (Hontoria et al., 1999;Paruello, 1998).The results showed differing trends.Although it was expected that SOC would increase with temperature decreases, SOC decreased in southern Oregon (Homann et al., 1995) and Finland forests (Liski and Westman, 1997).These different trends were caused by climate regimes, seasonal weather differences, altitude differences and other factors (Homann et al., 2007).
Biomass accumulates as a result of the autotrophic synthesis of organic compounds on live plants.These processes show that live and dead organic materials contain C and N elements.Because of this relationship, the C:N ratio has been used to characterise live and dead organic matter.Total C accumulation in biomass is generally limited by the effective N amount (Melillo, 1981;Aber et al., 1989).If the effective N increases, biomass increases and therefore C fixation increases (Mäkipää et al., 1999).
In studying ecosystem stability, it is important worldwide to determine C:N ratios and create data banks of the results, because this ratio serves as an indicator of stability.The goal of this study was to determine the C and N stocks, the C:N ratios and the relationship between C and N in the Harran Plain soils in Turkey.

MATERIAL AND METHODS
The Harran Plain is located in the Southeast Anatolia Region, between 38º 48' to 39º 12' E longitude and 37º 09' to 36º 42' N latitude, and it covers 225,000 ha in area.Arid and semi-arid soils were opened to irrigation there in 1995.In general, the Harran Plains descend to the south; one plain, whose three sides are surrounded with mountains, creating a pot-like appearance, is depicted in figure 1.The area between Harran and Akcakale has the lowest elevation.Dinc et al. (1988) detected 25 soil series on Harran Plain.
According to the studies of the Soil Survey Staff (1975), FAO/UNESCO (1974) and Dinc et al. (1988), the plain soils are of the Entisol, Vertisol and Aridisol soil orders.The soil group details of the Harran Plain soils are provided in table 1.Studies showed that the Bellitas (5), İkizce (7) and Cekcek (2) series are of the Entisol order; the Bozyazı (12), Ugurlu (1), Begdes (10), Akcakale (15) and Kısas (4) series are of the Vertisol order; and the Gurgelen (6), Ekinyazı (14), Akoren (13), Irice (9), Harran (11), Kap (16), Sultantepe (3) and Sırrın (8) series are of the Aridisol soil order.2).Specifically, while precipitation is low in Akcakale, the southernmost point, it reaches up to 450 mm on the northward foot of the Sanliurfa mountains.The soil moisture regime of important parts of the plain is Xeric, and the temperature regime is Mesic.Specifically, an Aridic soil humidity regime is seen in parts of the areas near the south of the plain (Soil Survey Staff, 1996).
In addition to being deficient in rainfall during most of year, the plain also experiences high temperatures for long periods of time.As table 1 shows, there is little precipitation between May and October, and temperature and evaporation are high when rainfall is low.The evaporation rate increases from the foot of the Urfa mountains to Akcakale.The seasonal average precipitation is highest in the winter months and lowest in the summer season.
Soil samples were taken from 16 series of genetic horizons on the Harran Plain.The soil samples were dried at room temperature and passed through a 2 mm sieve prior to analysis.The bulk density (BD) (Mg m −3 ) was determined according to Black (1965).The organic carbon content was estimated by titrating the samples boiled with sulphuric acid and Fe 2 SO 4 (Walkely and Black, 1934).The SOC (kg C m −2 ) stock was calculated according to Batjes (1996).The nitrogen content was analysed using the micro-Kjeldahl method (AOAC, 1990).The samples were read on a device that was set at 850 ºC (FP 526 LC, LECO).The total nitrogen stock (Kg N m −2 ) was computed with the method that was used for calculating the SOC stock.Student's t-test was used on all of the data comparisons and equation determinations and the data were examined at p<0.05 and p<0.01 significance levels.

RESULTS
The SOC amounts and stocks, the N amount and stocks, and the BD were determined at 100 cm depths of Harran Plain soils.The bulk density varied between 1.23-1.34Mg m -3 (Table 3).

Soil organic carbon stocks
The SOC content was at its lowest in the Harran series (0.80 kg C m −2 ) and at its maximum level in the Sırrın series (1.85 kg C m −2 ).The total soil carbon (SOC) stocks varied between 0.80 and 1.85 kg C m −2 .The carbon amounts of the other series varied significantly (p<0.05) and are shown in table 4. The soil organic carbon stock of the Harran Plain was 10.53 Tg C. The organic carbon contents were higher on the northern side where precipitation was higher.It is commonly known that, in general, temperature decreases as precipitation increases.High temperatures generally accelerate organic matter decomposition; hence, SOC decreases.Whereas the precipitation amount was 277 mm in the southern region, it was nearly 450 mm in the north.Therefore, carbon amounts were lower in the southernmost Akcakale series than in the northern Sırrın series.The carbon content of the plain was higher than that expected of an arid or semi-arid region because of increased soil depth, movement and accumulation of surface materials from high areas to the plain, high clay content (45-73%), too much calcareousness, and the constant rejuvenation of the plain soils.

Total nitrogen stocks
The total nitrogen contents were between 0.16 and 0.34 kg N m −2 , with the lowest content occurring in the Akcakale series and the highest content occurring in the Sirrin series.The nitrogen amounts of the other series are represented in table 5.The total N stock was 1.96 Tg N on the Harran Plain.The nitrogen content, like the carbon content, was higher in the northern profiles than in the southern profiles.It is hypothesised that the reason for this is higher precipitation.Although it is known that the Harran Plain soils have high clay content, the effects of clay on nitrogen stocks are not known.There are no studies examining the texture-nitrogen relationship on the Harran plain.Generally, concentrations of nitrogen are high in areas where the SOC is high.This shows a positive C:N relationship.According to this relationship, clay decreases SOC oxidation.Hence, it is hypothesised that there could be a positive relationship between clay content and nitrogen.Moreover, there is little effect of temperature and humidity parameters affecting the effcects of carbon stocks on nitrogen stocks.However, to explain their relation with nitrogen, guesses are done related to their effects on carbon.

Carbon:nitrogen (C:N) ratios
The carbon:nitrogen ratios were important (p<0.01) in all series throughout the profile (100 cm).The average C:N ratios of all of the series were ordered as follows: Sırrın › Irice › Bellitas › Ikizce › Begdes › Ugurlu › Gurgelen › Ekinyazi › Akoren › Kısas › Sultantepe › Akcakale › Kap › Bozyazi › Cekcek › Harran (Table 6).The C:N ratios were generally high in areas where the altitude, the vegetation and the precipitation were high.The C:N ratio was similar in the plain soils.This shows that the resolution and separation amounts are high.Moreover, the application of too much nitrogen fertiliser may have narrowed this ratio.A highly significant relationship existed between carbon and nitrogen contents (r = 0.9973; p<0.01) (Figure 2).
The C:N ratios ranged between 4.32:1 and 6.04:1.There was not much variation among the C:N ratios of the plain soils, which may be due to the similar climatic conditions and the agriculture management techniques Number 28/2011 ROMANIAN AGRICULTURAL RESEARCH adopted by the farmers.However, the small differences in the C:N ratios may also be due to variations in the microclimatic conditions, especially the temperature and the quantity and distribution of precipitation.

DISCUSSION
In this study, a small difference was observed between SOC and total N stocks and C:N ratios of Harran Plain soils in the northto-south direction.Average carbon stocks were slightly higher on the northern side than on the southern side, which may be due to higher precipitation on the northern portion than on the southern portion of the plain (450 mm vs. 277 mm).Furthermore, high precipitation caused temperatures to decrease to some extent.This corroborates the observation that the SOC content decreases with increases in the annual temperature, as reported by Post et al. (1982), Tremblay et al. (2002), Ganuza and Almendros (2003), Lemenih and Itanna (2004), Wang et al. (2004) and Sakin (2010).According to Yimer et al. (2006) and Sakin et al. (2010a,b,c), the SOC stocks increase based on annual precipitation and biomass amounts and decrease relative to temperature.Soil C and N stocks are affected by climate (Post et al., 1982).In line with several earlier studies (Bationa and Buerkert, 2001;Yimer et al., 2006;Moges and Holden, 2008;Fu et al., 2010), a very strong relationship between C and N was observed in this study.
It is hypothesised that clay and calcareous plain soils retain high amounts of carbon and nitrogen, but there are different theories regarding the effect of the clay concentration in soil on the SOC accumulation.An increase in maximum and average SOC contents with increased soil clay content was reported from a few Great Plains sites (Nichols, 1984;Burke, 1989).However, this phenomenon cannot be generalised, as other factors like soil aluminum, extractable allophone content or specific surface area can also influence the SOC content (Percival et al., 2000;Krull et al., 2003).The relationship between the clay concentration and the SOC content is strong when they are compared in soil organic matter (SOM) models like Century (Parton et al., 1987) and RothC (Jenkinson, 1990), which state that the SOM resolution decreases when the clay concentration increases.Wang et al. (2003) explained that clay had no effect on the first stage of soil occurrence, but that it can be effective at the later stages.Muller and Hoper (2004) reveal the different effects of clay on carbon resolution.McLauchlan (2006) argued that no strong relationship between clay and carbon resolution was observed.
The total N amounts and stocks are high on the northern portion of the Harran plain.The high nitrogen content is probably due to the high SOC content.The main reason for this is higher precipitation; Ganuza and Almendros data (2003)  The clay content of the plain soil is high.
Although the effect of clay on nitrogen stocks and amounts is unknown, a positive relationship was predicted.Some studies (Cote et al., 2000) state that the net N mineralisation decreases when the clay amount increases in the soil, but other studies (Giardina et al., 2001) found that the effect of clay on the net N mineralisation was low under different temperature and humidity conditions in the laboratory.McLauchlan (2006) explains that when clay amounts increase in soil, aggregate amounts increase dramatically and the potential net N mineralisation decreases.Whatever the age of the field, each 10% increase in clay concentration increases the aggregate size index by 0.039 and decreases the net N mineralisation by 0.16 kg ha −1 day −1 .The total nitrogen range 0.16 to 0.34 kg N m -2 in 100 cm soil depth (Table 5) observed was similar to those observed in several previous studies.Carter et al. (1998) reported a total nitrogen range of 0.36-1.05kg N m −2 in Canada farming soils.Zinke and Stangenberger (2000) found 0.61 kg N m −2 in Sierra shallow cone forests and 0.27 kg N m −2 in Nevada forests.Other nitrogen ranges observed include 0.5 kg N m −2 in mineral soils (Vejre et al., 2003), 0.21-3.13kg N m −2 in Amazon soils (Batjes and Dijkshoorn, 1999), 1.39 kg N m −2 in Podzol (Spodosol) soils, 1.03 kg N m −2 in Luvisoller (Alfisol) soils, 0.52 kg N m −2 in Arenosoller (Entisol) soils (Batjes, 1996), 0.17-0.29 kg N m −2 (Fu et al., 2010) and 0.05-1.65 kg N m −2 (Callesen et al., 2007).According to Callesen et al. (2007), N was higher in calcareous soils (1.12 kg N m −2 ) than in fine-textured soils (0.62 kg N m −2 ) and medium-and coarse-textured soils (0.51 and 0.48 kg N m −2 ).In the research area, the Bellitas, Ikizce and CekCek series (Entisol) included, respectively, 0.81, 0.88 and 0.93 kg N m −2 ; these amounts are high when compared with the Batjes (1996) studies and normal when compared with Batjes and Dijkshoorn (1999) study.It is hypothesised that this is based on high precipitation and soils that are calcareous and contain too much clay.
The C:N ratio in the surface soil was higher than that in lower portions of the subsurface soil horizons.This indicates high resolution and separation rates.Furthermore, it is thought that extreme cultivation techniques affect the C:N ratios.The C:N ratios varied between 4.07:1 and 6.04:1 (Table 6).Lal et al. (1995) indicated that C:N ratios are low during resolution and separation times.Brady and Weil (2008) showed that C:N ratios varied between 8:1 and 15:1, with an average of 12:1.Batjes (1996) determined (at a depth of 100 cm) that the lowest average C:N ratio was 7:1 in Xerosols and that the highest average C:N was 24.5:1 in Podzols.Although the C:N ratios in this study showed similarities to the Batjes (1996) findings, this study's ratios were lower.It is hypothesised that this phenomenon was caused by low precipitation, high resolution and separation rates and extreme cultivation techniques.Whereas the C:N ratio increases with precipitation, it decreases with higher temperatures (Miller et al., 2004).Other researchers argue that there is a positive relationship between C:N ratios, precipitation and temperature (Callesen, 2007).It is argued that although the cultivation systems and farming activities used 10 years ago did not affect C:N ratios (Sainju et al., 2008;Fu et al., 2010), today's farming techniques and agriculture do affect C:N ratios (Puget and Lal, 2005;Yimer et al., 2007).

CONCLUSIONS
Carbon amounts and stocks, N amounts and stocks and C:N ratios of plain soils are generally higher than similar environments.The reasons for the high C and N contents are precipitation, high clay and calcareous contents, soil depths, material movement from high areas to the plains and soil regeneration.The close C:N ratios are based on high resolution and separation amounts because of high temperatures, oxidation and fertiliser usage by farmers (which include high levels of nitrogen).

Figure 1 .
Figure 1.Location of the Harran Plain

a
Soil series are listed byDinc et al. (1988); b CV = the coefficient of variation (%); N = the number of observations.

Figure 2 .
Figure 2. Relationship between carbon and nitrogen

Table 3 .
Statistical a Soil series are listed by Dinc et al. (1988); b CV = the coefficient of variation (%); N = the number of observations.

Table 4 .
ERDAL SAKIN ET AL.: CARBON AND NITROGEN STOCKS AND C:N RATIOS OF HARRAN PLAIN SOILS Carbon amounts of Harran Plain soils (kg C m −2 ) a Soil series are listed by Dinc et al. (1988); b CV = the coefficient of variation (%); c Soil depth of 100 cm total SOC; N = the number of observations.

Table 5 .
Nitrogen content of Harran Plain soils (kg N m −2 ) a Soil series are listed by Dinc et al. (1988); b CV = the coefficient of variation (%); c Soil depth of 100 cm total N; N = the number of observations.

Table 6 .
C:N ratios of Harran Plain soils (kg m −2 ) c verify this theory.