Effect of Farmyard Manure Application on Dissolution of Carbonate Rocks and Its Eco-environmental Impact

Changli Liu*, Yun Zhang, Chao Song, Hongbing Hou Jianmei Jiang, Jingjing Wang, Lixin Pei

Institute of Hydrogeology and Environmental Geology,Chinese Academy of Geological Sciences, Shijiazhuang, 050061, China Corresponding author: Changli Liu, Email:liuchangli@vip.163.com,Phone:86-311-87704118； Tax: 86-311-87704118

Abstract: A much improved understanding of how farmyard manure application may affect carbonate rock dissolution is needed in order to predict possible feedbacks between the rock carbon cycle and the global climate system. Two carbonate mineral rock tablets； dolomite and limestone were buried at six depths between 0 and 110 cm in a soil typical of the subtropical karst area in Guiyang City, Guizhou Province. The extent of tablet dissolution, soil CO2, soil pH, soil water content, soil mineral and chemical composition, and chemical composition of soil water were tested in order to assess the degree of dissolution under manure application over the course of one year. The results show that manure addition decreases the dissolution rate of carbonate rocks； limestone and dolomite by between 11.7%-116.9% and 25.0%-65.69%respectively, with the dissolution rate of limestone consistently exceeding that of dolomite under the same conditions. Our data indicates that the rate of pedogenesis of the dolomite and limestone rocks is decreased as much as 35.77% and 59.41% respectively, as a result of manure application. Moreover, the results suggest that manure application accelerated the generation of soil CO2, with soil CO2 concentration increasing on average by 93.94%, and the CO2 flux increasing by 67.64% compared with the control profile.Finally, the data also indicates that manure decreases CO2 uptake by dissolution of carbonate rocks by 25.50%-39.45% on a Guiyang city scale. The counteraction of the CO2 sink contributed by karst water due to farmyard manure utilization in general karst area (both dolomite and limestone) however was 59.41%-62.72%, indicating the application of manure successfully reduces both dissolution and CO2 release to the atmosphere.

Keywords: atmospheric CO2 Source/Sink； carbonate rocks； dissolution； farmyard manure； pedogenesis

1. Introduction

Climate change, including changing temperatures, altered precipitation regimes and elevated atmospheric CO2concentration are threatening the future of the planet, as a result,climate change has attracted considerable attention from governments and academia all over the world.Rocky desertification and pedogenesis of carbonate rocks are two significant issues related to sustainable development of economy and ecological environment in karst mountainous area,which could be strongly influenced by a changing climate. Karst topography is characterized by the underlain carbonate rock, which through dissolution of the soluble layers creates a distinctive topographic formation.

China is the largest distributing region of carbonate rock in the world； the area of exposed carbonate rock of 8 provinces in Southern China is 51.9 km2(Datong Li 1983), where the area of infield forming by carbonate rock weathering is almost 10,0000 km2(Jianhua Cao 2004). The pedogenesis of carbonate rocks has attracted much attention in recent decades due to its close relation with the control and prevention of rocky desertification.

The universally accepted dissolution-residuum theory shows that soils in karst area form through the deposition of insoluble residues of carbonate rocks. Research over recent decades revealed that the pedogenesis rate is extremely low (Daoxian Yuan 1989； Mingde Yang 1990； Guojiang Wan 1998； Shijie Wang 1999). Such as the research of Daoxian Yuan (1989) in the karst area with high purity carbonate rock, the content of acid-insoluble matter is low, usually under 10%, and even under 1%, and their study in the subtropical area of Guixian in Guangxi showed that the forming of 1 cm soil layer needed the denudation of 25 m carbonate rock, confirming that pedogenesis is very slow in karst regions. Shijie Wang et al.（1999）reported that the weathering-pedogenesis rate of carbonate rocks was extremely low, which was in direct correlation with climate, lithology and the content of the acid-insoluble matter (i.e.lignin). Therefore, the relationship between fertilization in human cultivation and the pedogenesis of carbonate rock can be analyzed and assessed by the results of this research.

Karst process, as a process of atmospheric CO2consumption on the earth surface (Daoxian Yuan 1993a), and is of significance for study with regards to the global carbon circle and climate change (Brown 1994； Daoxian Yuan 1993b；Genxing Pan 2000). Many results on driving effect of soil CO2on karstification and the relationship between soil CO2and atmospheric CO2source/sink have been reported (Guojiang Wan 1995； Daoxian Yuan 1993a, 1997； Li Bin et al 1996； Shengyou Xu 1996, 1997； Leping Zheng 1999； Jianhua Cao 2005； Genxing Pan 2000),whereas there is little information on the relationship between the dissolution of carbonate rocks and atmospheric CO2source/sink in soil using farmyard manure.

In order to able to reduce CO2emissions and prevent the rocky desertification of the karst region,which will in turn protect the surrounding farmland,research is required to study the effect of soil CO2on the dissolution of carbonate rocks together with unraveling the cause and rate of soil formation and soil CO2fluxes in this region. Here we report our results on the effect of farm manure application on the dissolution of carbonate rock and its eco-environmental impact.

2. Materials and methods

2.1 Study area

The study was conducted within a typical subtropical karst area of SW China (HuaXi district of Guiyang city, Guizhou province), where the depth of residual soil of weathered carbonate rock ranges between 0 and 1.5 m and the mean annual temperature and rainfall is 14.9°C and 1229 mm,respectively. The soil type across the two sites was residual soil of dolomite weathering (Table 1).

2.2 Methods

The field experiment design consisted of two split soil profiles (manure profile (MP) and control profile (CP)) which were placed 50 cm from each other in the same plantation. At each profile site a carbonate rock tablet (1 cm x 2 cm x 0.5 cm) was buried at depths of 0 cm, 15 cm, 30 cm, 50 cm, 90 cm and 110 cm.

Soil CO2concentrations were monitored from gas wells established at corresponding depths in soil profiles. The gas wells consisted of 4 mm tubing inserted to the assigned depth in the soil and fitted at the surface with a stoppered, female end of a plastic union. The soil water samplers were also placed at the corresponding depth (Fig. 1). The two soil profiles were of consistent physical and biogeochemical characteristic before experiment.The plot profiles allow the effect of farm manure application on the dissolution of carbonate rock tablets to be compared both between rock and profile depths.

The manure utilised was a mixture liquid of horse and cattle manure with a pH range of 7.4 to 8.1. The quantity and frequency of manure were consistent with those of local farm.

2.2.1 Experiment site description

The field experiment site (Fig. 2) was established on a farm in a cabbage-corn or capsicum-corn rotation at the southwest of the Guizhou Province Committee Party School in 2006.

Fig. 1: The soil texture and the structure of field experiment site

Table 1: The soil properties at each experimental soil profile

2.2.2 Observation index

2.2.2.1 Extent of dissolution

The six tablets, consisting of 6 limestone tablets and 6 dolomite tablets were baked at 80°C for 8h and weighed. These tablets were buried at the six depths of the two soil profiles (manure profile and reference profile) in July 2007. The six tablets were subsequently carefully removed,rinsed and re-weighed on three occasions over one year； December 2007, July 2008 and December 2008.

2.2.2.2 Soil CO2and ground CO2flux measurement

The measurements of soil CO2and ground CO2flux were carried out on site. The frequency of measurement was weather dependant, but generally measured monthly. Soil CO2was measured with ATX620 (Industrial Scientific corp,Oakdale, PA, USA ) and ground CO2flux was tested by static chamber method with alkali absorption.

2.2.2.3 Observation of factors

The principal factor affecting the dissolution of carbonate rocks and soil CO2is pH. Soil pH was measured monthly on site with an IQ pH6 Meter(IQ Scientific Instruments, Inc., Carlsbad, US ) and soil temperature was observed quarterly on site.Other relative indexes such as soil mineral and chemical composition, soil physical characteristic,chemical composition of soil water, manure component were tested quarterly at a qualified company or laboratory. The weight of rock tablets,the depth, soil pH are shown in Table 2.

Table 2: The general situation of field experiment design

3. Results & Discussion

Table 3: The results on annual dissolution amount of limestone and dolomite tablets and soil CO2

3.1. Impact of manure application on limestone and dolomite dissolution

The changes in the extent of dissolution in the limestone and dolomite tablets are displayed in Table 3, Fig. 3 and Fig.4. These results show that at the same depth, the extent of dissolution in the limestone and dolomite tablets in the CP consistently exceeded that shown in the MP. The dissolution amount of limestone tablets was more than that of dolomite tablets (Table 3). Dissolution of the limestone carbonate rock is decreased by between 11.7% and 116.9% (average 34.2%) and between 25.0% and 65.69% (average 39.45%)after the application of manure. The decrease of dissolution in the MP was shown in Figs 7 and 8 to be related to an increase of soil pH (Table 3； Fig. 7,Fig. 8). Although relative results reported that the karst system is of a higher capacity of dissolution under a higher CO2concentration, our data (Table 3, Fig. 7, Fig. 8) showed that an elevated soil CO2did not increase the capacity of dissolution of carbonate rock because the solubility of CO2in soil water is limited.

Fig. 3 Dissolution comparison of dolomite tablets in manure profile with in control profile

Fig. 4 Dissolution comparison of limestone tablets in manure profile with in control profile

3.2. Impact of manure on soil CO2 and ground CO2 flux

Fig. 5 and 6 show that manure application accelerates the generation of soil CO2, ground CO2flux, and soil CO2concentration. The soil CO2concentration in the manure soil profile increases between 22.52% and 198.87% (average 93.94%)compared with control soil profile and the CO2flux increases 67.64% this is explicable by an increase of SOM.

Fig.5: Comparison of soil CO2 concentration in manure profile

Fig.6: Comparison of CO2 flux in manure profile with in control profile

Fig.7: The relationship among the dissolution of carbonate rocks, soil CO2 and pH in control profile

Fig. 8: The relationship among the dissolution of carbonate rocks, soil CO2 and pH in manure profile

3.3. Implications for eco-environment

3.3.1 Impact on the pedogenesis rate of carbonate rocks

The dissolution-residuum theory has been confirmed in karst areas, for example； Demin Yang (1990) showed the average weathering rare of carbonate rock was 61.8 mm/ka, with a corresponding pedogenesis rate of 0.31-2.47 mm/ka. In addition, the research of Guojiang Wan(1998) in the karst area of central Guizhou showed the physical dissolution rate of carbonate rock was 0.003 g/(cm2·a) (about 11.1 mm/ka)，but the building rate of weathering deposits was lower than 0.001 g/(cm2·a) (about 3.7 mm/ka).Furthermore, Shijie Wang et al.（1999）reported that the weathering-pedogenesis rate of carbonate rocks was extremely low, which was in direct correlation with climate, lithology and the content of the acid-insoluble matter (i.e. lignin).

This influencing strength is defined as“reduced soil production in unit area of fertilized carbonate rock” divided by “soil production in unite area of unfertilized carbonate rock” in unit time, and expressed by β, then:

where G0=Erosion in unit area of carbonate rock in unfertilized profile, g/cm2； G1=Erosion in unit area of carbonate rock in fertilized profile g/cm2. The influencing strength of farmyard manure utilization on the different thickness soil on the pedogenesis rate of underlying carbonate rock was calculated using equation 1 and the data in table 3, with the results shown in table 4.

Table 4: The influencing strength of farmyard manure utilization on the pedogenesis of carbonate rock

Table 4 shows that farmyard manure utilization has a distinct influence on carbonate rock pedogenesis. Under the same conditions, the influence on limestone was larger than that on dolomite (Table 4). The influencing strength of dolomite ranged from 25% at 36 cm to 65.7% at 20 cm, while that of limestone ranged from 11.7% at 90 cm to 116.9% 20 cm；both showing maximum influence at 20 cm. The influence of farmyard manure on carbonate rock pedogenesis was not only affected by soil pH, but also affected by other factors (e.g. soil character, water content and permeability), however the impact of these factors decrease with the increase of soil thickness.

In the carbonate rock area of Southwest China,most of the soil thicknesses are bellow 50 cm(Mingde Yang 1988), which is the same depth of furrowed layer. From our study, the influencing strength on dolomite and limestone pedogenesis by farmyard manure utilization at 50 cm depth are 35.77% and 59.41%, respectively, these two numbers has regional significance (Table 4).

Farmyard manure utilization has distinct influence on carbonate rock pedogenesis, thus,adjustment of local industrial structure,improvement of the cultivation and fertilization type may be a good choice of soil conservation and ecological regulation.

3.3.2 Impact on atmospheric CO2source/sink

The study of the carbon isotope trace indicated that a large contribution of atmospheric CO2to soil CO2existed in the soil system in karst area, with a considerable exchange between atmospheric CO2and soil CO2. However, soil CO2was several dozen times higher than that of atmospheric CO2(Guojiang Wan 1995). Our results showed that soil CO2ranged between 707 and 3,850 ppm (Fig.5)which is significantly higher than the atmospheric CO2(190 ppm). Thus the direction of CO2movement was as a whole, from the soil to atmosphere. This direction of movement will further increase atmospheric CO2levels, which will further increase climate change. The soil CO2concentration in the MP increased from between 22.52% and 198.87% compared with the CP and the CO2flux increased 67.64%.

Carbonate rock dissolution is a sink for the soil system because parts of soil CO2dissolved in soil water can cause a decrease of pH, as a result more carbonate rocks are dissolved. The efflux of CO2from the soil to atmospheric was decreased because of the consumption of soil CO2by carbonate rocks dissolution. In other words, the dissolution of carbonate rocks is an indirect sink of atmospheric CO2. The change in soil condition such as a result of farmyard manure utilization,caused an increase in the soil pH (Table 2),consequently, the dissolution amount of carbonate rocks was decreased (Table 3). As a result, the soil CO2which should have been consumed by carbonate rocks fluxed to the atmosphere.Therefore, although the dissolution of carbonate rocks was a sink of atmospheric CO2, farmyard manure utilization was regarded as an indirect source of atmospheric CO2due to the decreasing extent of dissolution of carbonate rocks.

As a result of the decrease of carbonate rocks dissolution by farmyard manure utilization, CO2consumption was decreased, the decrease was calculated by the data in Table 3 and following chemical equation (2).

Where MeCO3was carbonate rocks in the real karst system, such as CaCO3or MgCO3. The calculated results are displayed in Table 5.

Table 5: The calculated results of the decrease of CO2 uptake by carbonate rocks weathering due to farmyard manure utilization for 1 year

Table 5 displays a decrease of CO2uptake by dolomite and limestone due to farmyard manure utilization. Within the dolomite rock, there was a decrease of 1.1704×104kg/km2, whilst the limestone rock displayed a higher decrease of 1.2358×104kg/km2. Expressed as a percent decrease of CO2uptake by dolomite and limestone due to farmyard manure utilization, there was a range from a 25.50% decrease in the limestone to a 39.45% decrease in the dolomite.

Karst water is a huge sink of CO2, and the calculated result of CO2consumption contribution to karst water is 47.18×109kg/a based on the distribution area of carbonate rocks (Lizheng Yang 2002). The decrease of CO2sink due to the decrease of carbonate rocks weathering caused by farmyard manure utilization would counteract the CO2sink contributed by karst water. Here we calculated how much the counteraction was accounted for. Relative data by Lizheng Yang(2002) indicated that CO2uptake by karst water in the Guiyang area was approximately 0.62g/s(1.97×104km2/a). The decrease of CO2consumption contributed by dolomite and limestone dissolution due to farmyard manure utilization was 1.17×104kg/km2a-1and 1.24×104kg/km2a-1, respectively. Hereby, the percentage of the decrease of CO2consumption contributed by dolomite and limestone due to farmyard manure utilization in Guiyang area was 59.41% and 62.72%, respectively.

4. Conclusion

The main results obtained in this study indicate that soils treated with manure reduced the degree of dissolution of carbonate rock, however the degree of reduction is dependent on the type of carbonate rock. Although dissolution is reduced after manure treatment, the same treatment accelerates the generation of soil CO2, which could result in increased CO2fluxes to the atmosphere.However, the decreased CO2consumption of the rocks and the increased soil CO2due to manure application can be counteracted by the effect of karst water. The counteraction percentage of the CO2sink contributed by karst water due to farmyard manure utilization in general karst area ranged from 59.41% to 62.72%.

The influencing strength of farmyard manure utilization on carbonate rock pedogenesis in the Guiyang area of exposed carbonate rock is between 35.77% and 59.41%.

In this study, the greenhouse gas CO2, which can greatly affect climate change, karstification and pedogenesis is of significant importance for global climate change, plantation protection at karst mountainous area and the regulation of eco-environment. However, due to the short term experiment carried out in this study in Guiyang,only some primary results were obtained. To acquire regional results, more extensive studies must be carried out in succession. We suggest that further studies need to be done in other typical areas, such as Guangxi, Yunnan, Sichuan,Chongqing, Hunan and Northern China.

5. Acknowledgements

This study is supported by National Basic Research Program of China (973 Program)(No.2006CB403200). The efforts of academician Yaoru Lu are greatly appreciated for his care for our research and kindly help!