**Muhammad Shaaban 1,2, Yupeng Wu 1, Lei Wu 3, Ronggui Hu 1,\*, Aneela Younas 4, Avelino Nunez-Delgado 5,\*, Peng Xu 1, Zheng Sun 6,7, Shan Lin 1, Xiangyu Xu <sup>1</sup> and Yanbin Jiang <sup>1</sup>**


Received: 19 April 2020; Accepted: 15 June 2020; Published: 17 June 2020

**Abstract:** Nitrous oxide (N2O) is an overwhelming greenhouse gas and agricultural soils, particularly acidic soils, are the main source of its release to the atmosphere. To ameliorate acidic soil condition, liming materials are added as an amendment. However, the impact of liming materials has not been well addressed in terms of exploring the effect of soil pH change on N2O emissions. In the present study, a soil with pH 5.35 was amended with liming materials (CaMg(CO3)2, CaCO3, Ca(OH)2 and CaO) to investigate their effects on N2O emissions. The results indicate that application of liming materials reduced the magnitudes of N2O emissions. The maximum reduction of soil N2O emissions took place for Ca(OH)2 treatment when compared to the other liming materials, and was related to increasing soil pH. Mineral N, dissolved organic C, and microbial biomass C were also influenced by liming materials, but the trend was inconsistent to the soil pH change. The results suggest that N2O emission mitigation is more dependent on soil pH than C and N dynamics when comparing the different liming materials. Moreover, ameliorating soil acidity is a promising option to mitigate N2O emissions from acidic soils.

**Keywords:** lime; mineral nitrogen; soil pH; organic carbon; microbial biomass; N2O

### **1. Introduction**

Soil acidity is a master variable that hinders plant growth by limiting nutrient availability and thus impacts both the quantity and quality of crops. Soil acidification occurs very slowly naturally as soil is weathered, but this process is accelerated by intensive agriculture [1,2]. Soil acidity is expressed in terms of pH, and its extent and degree impact a wide range of soil biogeochemical properties. Soil acidity also has marked effects on soil microbial communities and their pertinent processes. Soil acidification is a natural and very slow process that takes over hundreds of years to develop. However, it may reach its greatest expression within a few years under intensive agricultural practices

and in humid regions where rainfall is sufficient to leach down the nutrients [3]. Thus, although most processes developing soil acidification are natural, anthropogenic activities have a major impact on some of them. In fact, several reasons may contribute to soil acidification and excessive use of nitrogen (N) is one of them [3].

To obtain high crop production in intensive crop-growing areas, excessive application of N fertilizers has been carried out for years, but when it is excessive, it leads to soil acidification [4]. According to estimations [3], the application of nitrogen fertilizer in arable lands of China usually ranges from 200 to 500 kg N ha−<sup>1</sup> yr<sup>−</sup>1. Aside from the beneficial effects of high N fertilizer application, devastating impacts and environmental risks have also been observed including eutrophication, nitrous oxide (N2O) emissions, and soil acidity [5,6]. Researchers have demonstrated that nitrate and ammonium applied to soils can generate 20 to 33 kmol hydrogen ions (H+) ha−<sup>1</sup> yr−<sup>1</sup> under exhaustive growing systems [3]. This indicates that the application of N can drive soil acidification.

Soil acidity can be offset with alkaline materials that provide conjugate bases such as CO3 <sup>2</sup><sup>−</sup> and OH<sup>−</sup> of weak acids. These anionic bases react with H<sup>+</sup> and form weak acids. For example:

$$\rm{CO\_3}^{2-} + 2\rm{H}^+ \rightarrow \rm{H\_2CO\_3} \tag{1}$$

Generally, liming materials are applied in the forms of hydroxides or oxides containing magnesium (Mg) or calcium (Ca), which form hydroxide ions in water.

$$\rm CaO + H\_2O \to Ca(OH)\_2 \to Ca^{2+} + 2OH^- \tag{2}$$

Most liming materials, whether they are carbonates, hydroxides, or oxides, react with CO2 and H2O to generate bicarbonates (HCO3 <sup>−</sup>) when added to acidic soils. As a result, partial pressure of CO2 in the soil is high enough to proceed such reactions forward, for instance:

$$\text{CaMg(CO}\_3\text{)}\_2 + 2\text{H}\_2\text{O} + 2\text{CO}\_2 \rightleftharpoons \text{Ca}^{2+} + \text{Mg}^{2+} + 4\text{HCO}\_3^-\tag{3}$$

The resultant bicarbonates, Ca and Mg, counteract the acidity.

Liming acidic soils not only raises soil pH, but also alters biochemical processes and nutrient cycling. The rise in soil pH following lime application substantially triggers the N transformation processes [7], markedly controls the microbial processes of nitrification and denitrification, and thus influences N2O production and emission. However, the subsequent effects of lime application on N2O emissions is ambiguous and contrary hypotheses have been proposed by the scientific community. For instance, a laboratory incubation study proposed that increasing soil pH may substantially decrease emissions of N2O from acidic agricultural soils [8]. In contrast, some scientists have reported that lime application and subsequent rise in soil pH caused increased soil N2O emissions from arable acidic soils [9,10].

Keeping in mind the importance of liming acidic soils, we hypothesized that the application of lime materials can trigger N transformations following soil pH change and subsequently influence N2O emissions in a way that would be interesting to further elucidate. Thus, the current study was conducted with the aim to examine and shed further light on the pH change effects of various liming materials on N2O emissions from acidic agricultural soils.

### **2. Materials and Methods**

### *2.1. Soil and Liming Materials*

Soil was obtained from a rapeseed-rice cropping system, located in Xianing (a city of central China; 29◦88 209 N, 114◦39 416 E). According to Soil Survey staff [11], the soil is classified as Ultisol. Soil (0–20 cm) was sampled from the selected field after rice harvest from multiple-points. A composite soil sample was made by mixing subsamples. Plant residues (straw and roots) were separated from

soil. After shifting in the laboratory, soil was dried in the open air, crumbled, and then sieved through a 2 mm sieve. The basic soil chemical and physical analysis [12] was performed prior to onset of the experiment. Soil texture was silty clay loam. The main characteristics of soil are given in Table 1. Different liming materials (dolomite CaMg(CO3)2, calcium hydroxide (Ca(OH)2, calcium carbonate CaCO3, and calcium oxide CaO) used in the present study were purchased from Xinjing Chemicals Co. Ltd. (Xiaogan, Hubei, China).


**Table 1.** Some selected physical and chemical characteristics of the tested soil.
