Influence of lImIng on ResIdual soIl RespIRatIon and chemIcal pRopeRtIes In a tRopIcal no-tIllage system ( 1 )

Because of the climate changes occurring across the planet, especially global warming, the different forms of agricultural soil use have attracted researchers ́ attention. changes in soil management may influence soil respiration and, consequently, c sequestration. the objectives of this study were to evaluate the long-term influence of liming on soil respiration and correlate it with soil chemical properties after two years of liming in a no-tillage system. a randomized complete block design was used with six replications. the experimental treatments consisted of four lime rates and a control treatment without lime. two years after liming, soil co2 emission was measured and the soil sampled (layers 0–5, 5–10, 10–20, and 20–30 cm). the p, ca2+ e mg2+ soil contents and ph and base saturation were determined. co2 emission from soil limed at the recommended rate was 24.1 % higher, and at twice the recommended rate, 47.4 % higher than from unlimed soil. liming improved the chemical properties, and the linear increase in soil respiration rate correlated positively with the p, ca2+ and mg2+ soil contents, ph and base saturation, and negatively with h + al and al3+ contents. the correlation coefficient between soil respiration rate and chemical properties was highest in the 10–20 cm layer.


IntRoductIon
the expected population increase in the coming decades indicates directions that seem paradoxical: more food and energy production are needed together with environmental protection.Reports of the Intergovernmental panel on climate change (Ipcc) emphasized the need for adaptation in several production areas, including latin america, as higher temperatures and drier periods are expected (Ipcc, 2007).therefore, in the near future, agricultural production should be adapted to the changing climate and, if possible, help mitigate atmospheric greenhouse gases, above all co 2 (Bernoux et al., 2005;cerri et al., 2007).
Because of the climate changes across the planet, particularly global warming, the different forms of agricultural soil use have attracted researchers´ attention (Batjes, 1998;adachi et al., 2006;costa et al., 2008;mclain & ahmann, 2008;li et al., 2009).according to li et al. (2009), changes in soil management may influence soil respiration and, as a consequence, drastically affect c sequestration. the increase of co 2 concentration in the atmosphere is partially due to respiration from the microbial activity in the soil (Ipcc, 2007).
In the huge regions with acidic soils in Brazil, liming is commonly used to correct soil acidity, which can represent an important source of co 2 efflux (Bernoux et al., 2003;amaral et al., 2004).some studies have shown that liming is one of the greatest factors of gas emissions that are responsible for global warming (Bernoux et al., 2003;cerri et al., 2006).this is caused by the increased co 2 flow to the atmosphere, as a result of caco 3 hydrolysis reactions in the soil.additionally, microbial activity, intensified by the improved soil chemical conditions after liming, has to be considered (chan & heenan, 1998; adachi et al., 2009).according to the Ipcc (2007), soil respiration associated with liming practices is estimated based on the limestone composition and the annually applied lime rate.for this estimation, the amount applied is multiplied by the emissions factors, which results in the co 2 amount released from the soil to the atmosphere.Based on the amount of limestone used in Brazilian agriculture from 1990 to 2000, the average annual liming-related soil respiration was estimated at 7.2 tg co 2 (Bernoux et al., 2003(Bernoux et al., , 2005)).however, this value would be underestimated, if the chemical reactions and the residual effects of liming lasted longer than a year.the objectives of this study were to evaluate the long-term influence of liming on soil respiration and correlate it with soil chemical properties two years after liming in a no-tillage system.

mateRIal and methods
the experiment was conducted in Jaboticabal,sp,Brazil (21° 15' 22'' s,48° 18' 58'' W;595 m asl).the climate, according to Köppen, is aw (tropical climate with a dry season), with a mean annual rainfall of 1,425 mm, mostly concentrated between october and march.the average annual temperature is 22 °c and average relative air humidity 70 %.
until 1995, the field had been used for corn (Zea mays l.) and soybean (Glycine max l. merrill) rotation under conventional tillage for more than 10 years.from 1995 to august 2000, the field was left fallow.In september 2000, the management was changed to no-tillage corn-soybean rotation, left fallow in the fall/winter.a randomized complete block design was used with six replications.the experimental treatments consisted of four lime rates and a control without lime, on 20 m 2 plots.lime was applied at 0.9, 1.8, 2.7 and 3.6 mg ha -1 (i.e., 0.5, 1.0, 1.5 and 2.0 times the limestone requirement to raise the soil base saturation to 60 %), determined based on the soil chemical analysis and the calcium carbonate equivalent (cce) = 100 %. the soil surface was limed by hand, without moldboard plowing on 1 sept, 2000.two years after liming (september 2002), soil respiration emission was measured in the previous crop inter-rows.emissions were recorded using a soil chamber (internal volume 991 cm 3 , exposed area of 71.6 cm 2 ) manufactured by the lI-coR company (lI-6400-09, lI-coR, ne, eua) coupled to a photosynthesis analysis system. the chamber was placed on the top of pVc plastic rings (diameter 10 cm) and installed on the soil one day before the readings.the chamber was coupled to the portable photosynthesis analysis system (lI-6400), which analyzes the internal co 2 concentration by optical absorption spectroscopy.Before each reading, the co 2 concentration within the chamber was reduced to 370 mmol mol -1 (370 ppmv) by passing the air through soda lime for a few seconds.
after the chamber was installed over the plastic rings, the soil co 2 emission was measured every 2.5 s, and the total co 2 emission was calculated after 90 s, which is when the concentration of co 2 in the interior of the chamber reached 390 mmol mol -1 .at the end of the measurement phase, a linear regression between co 2 emissions and co 2 concentration in the inner chamber was determined.this regression equation was then used to calculate co 2 emissions from the soil for a concentration of 380 mmol mol -1 in the chamber, which was the co 2 concentration in the air near the soil measured at the beginning of each set of measurements.the measurements were carried out in triplicate on each experimental plot.
after the respiration measurements, soil samples were obtained from each plot (layers 0-5, 5-10, 10-20 and 20-30 cm).six samples per plot were mixed to a composite sample for each soil layer.after sampling, the air-dried soil was sieved (2 mm). the total c, ph (cacl 2 ), resin-p, exchangeable K + , ca 2+ , mg 2+ and al 3+ , h + al, cec, and base saturation were determined for each sample, according to standard procedures described by page et al. (1982).soil respiration data were statistically analyzed (anoVa).due to the significance of results (p < 0.10), the effects of lime rates were subjected to polynomial regression analysis (p < 0.10).correlation analyses (p < 0.10) were carried out to detect significant functional relationships between soil respiration and chemical properties.

Results and dIscussIon
the results showed significant effect (p < 0.10) of lime rates on soil respiration, even two years after liming.soil respiration increased linearly with increasing lime rates (figure 1).It is worth noting that each soil respiration value was obtained in 18 measurements, ensuring a high degree of representativeness.the angular coefficient of the adjusted model (0.045), which represents the sensitivity of soil co 2 emission, demonstrated clear differences among the treatments: zero lime, recommended rate (1.8 mg ha -1 ) and twice the recommended rate (3.6 mg ha -1 ). the highest lime rate increased soil respiration by 47.4 % compared to the unlimed treatment and by 24.1 % compared with the recommended rate, as adequate for plants (lime requirement to raise base saturation to 60 %).
Results show that the liming rates applied to the soil directly affected soil respiration, even two years after liming.
the Ipcc method to calculate the liming-related soil respiration is based on the lime amount used in one year multiplied by emission factors, which depend on the chemical composition of the limestone.then, the soil co 2 effluxes are represented in released co 2 mass units related to the lime amount applied per year (Ipcc, 2007).consequently, the soil respiration due to the residual effect of liming (after more than a year) is not taken into consideration by the Ipcc method.therefore, the results obtained in this study indicate that the Ipcc method may underestimate soil respiration because the limestone, after two years, kept on reacting in the soil, and its effects lasted longer than considered by the Ipcc.
according to fuentes et al. ( 2006), the fast co 2 release from the soil after liming is a result of two parallel processes: a chemical process, as consequence of the caco 3 hydrolysis reaction in the soil; and a biological process caused by the increased microbial activity in the soil, due to the improvement of soil chemical conditions.In the present study, probably, the major contribution to soil respiration increase was the result of biological processes, because liming was carried out two years before the soil respiration measurements.liming improved the chemical conditions in the soil profile (figure 2), favoring microbial biomass activity and growth (haynes & swift, 1988;la scala Junior et al., 2000), which increased the microbial respiration rate and soil co 2 emission with increasing lime rates (figure 1).similar results were reported by fuentes et al. ( 2006), who stated that the increase of soil ph provided by liming affects the activity and microbial population in the soil.however, the authors pointed out that these effects depend on soil type and soil management.In acidic soils, neale et al. (1997) observed that liming improved chemical and environmental conditions in the soil, promoting the development of acid-intolerant microorganisms, which leads an increase in soil microbial biomass and respiration.In an evaluation of the population of bacteria after liming (7.5 mg ha -1 ) in acidic soil, shah et al. ( 1990) noticed a 20 fold increase in the amount of bacteria.however, the co 2 -releasing caco 3 hydrolysis reaction must be taken into account because correlations between soil chemical properties and respiration have been verified, even in the deepest soil layer.In the 0-5 cm layer, soil respiration correlated (p < 0.10) with p-resin (r = 0.42), ca 2+ (r = 0.40), h + al (r = -0.45)and base saturation (r = 0.38); in the 5-10 cm layer, it correlated with ca 2+ (r = 0.35) and al 3+ (r = -0.38); in the 10-20 cm layer, with ph (r = 0.43), ca 2+ (r = 0.41), h + al (r = -0.36),al 3+ (r = -0.35)and base saturation (r = 0.40); and in the 20-30 cm layer, with ca 2+ (r = 0.42), mg 2+ (r = 0.39) and al 3+ (r = -0.39).these values could be related to the downward movement of limestone, which occurred in the two years after application on the soil surface, while it kept reacting with soil at greater depths (figure 2), releasing co 2 from caco 3 hydrolysis.still, there is no doubt that at these depths, liming also improves the soil chemical conditions, favoring increased microbial activity (shah et al., 1990; nealy et al., 1997; fuentes et al., 2006).however, it is known that at lower depths the microbial population is much smaller and contributes less to soil respiration than in the topsoil.changes in the soil chemical conditions of subsurface layers, such as potential acidity reduction and increase of ph and ca 2+ and mg 2+ content, have already been shown in no-tillage systems (oliveira & pavan, 1996; mello et al., 2003; caires et al., 2006).these changes occur for the following reasons: 1) the physical downward movement of limestone particles, transported through bio-pores formed after the decomposition of dead roots (oliveira & pavan, 1996) or by soil organisms (mello et al., 2003), which remain intact due to the no-tillage farming system; 2) the downward movement of ca 2+ and mg 2+ ions bonded to no 3 -or so 4 2-(no 3 -or so 4 2-originated from fertilizer or were released by the mineralization of organic matter) (mello et al., 2003); 3) or even by the formation of water-soluble organic complexes present in plant residues left on the soil surface in no-tillage systems (franchini et al., 2001).these organic compounds, bonded with ca 2+ or mg 2+ , form cal 0 or cal -type complexes, making them more mobile in the soil.nevertheless, it is believed that a combination of these mechanisms is most likely to have occurred in the system studied here.this combination of mechanisms explains the movement of limestone and of its reaction products to the deeper layers, where it improves the soil chemical conditions or even undergoes hydrolysis, resulting in increased soil respiration in both cases.the correlation between soil p content and respiration from the 0-5 cm layer can be attributed to the increase in p availability at higher soil ph, as a result of liming (haynes & swift, 1988).the absence of al 3+ observed in the 0-5 cm layer (figure 2) is a result of the precipitation of al 3+ in the soil solution when the ph of the medium reaches values ≥ 5.5, eliminating aluminum in the chemical form al 3+ from the soil solution (mcBride, 1994).
there was no significant correlation between soil respiration and K + , organic matter content or cec.these results may be due to the fact that liming does not affect these soil chemical properties straightforwardly.liming does directly influence the soil ph and ca 2+ , mg 2+ , and h + al soil contents, which in turn affect microbial activity and, consequently, soil respiration.changes in the organic matter content occur slowly, and could not be detected in the present study because of the short period after liming.the same explanation applies to soil cec, which is directly linked to the soil organic matter content in tropical systems.conclusIons 1. the soil co 2 emission rate increased linearly with increasing liming rates in a no-tillage system even two years after liming.
2. When the recommended lime rate to raise soil base saturation to 60 % was applied, soil respiration was 24.1 % higher than from unlimed soil two years later, and 47.4 % higher when twice the recommended rate had been applied.

figure 1 .
figure 1. linear regression model for lime rates and soil co 2 emission, two years after liming without moldboard plowing.* significantly different (p < 0.10).error bars represent one standard error of the mean.