Estimating nitrogen leaching losses after compost application in furrow irrigated soils of Pakistan using HYDRUS-2D software

Highlights

• We model nitrogen (N) transport from compost amended soil under furrow irrigation.

• We examine N losses after application of press-mud and poultry manure composts.

• N mineralized faster form the press-mud composts and led to higher N losses.

• N losses were the highest in wet year due to N leaching from ridges by rainfalls.

• N leaching from the soil were reduced by doubling width of the ridges.

Abstract

Furrow irrigation used in arid areas for maize production is a major cause of groundwater pollution by nitrates, and development of new technologies aimed at reducing pollution is critical for sustainable crop production and water usage. Field experiments and modeling of nitrogen (N) transport were carried out to estimate N losses from compost amended soils under furrow irrigation used for maize production in Pakistan. Press-mud (PrM) and poultry manure (PM) composts were applied at 5 application rates followed by 8 irrigations at rates of 7.5 cm. Changes in inorganic N storage (NH₄N and NO₃N) were measured in the top 15-cm soil layer. The HYDRUS-2D model was calibrated and validated on measured data and implemented to estimate N leaching losses from soil profile. A four-pools approach used in the simulations considered transformation and transport of the following pools of N: ‘Slow soil N’, ‘Fast soil N’, ammonium-N, and nitrate-N pools. Results showed that N leaching losses were higher for: (i) PrM compost due to its higher mineralization rate; (ii) wet year due to washing out of N accumulated in ridges; and (iii) 1:1 ridge to furrow aspect ratio in comparison to the 2:1 ratio, due to an increase in N accumulation in wider ridges. Overall, the HYDRUS-2D software appeared to be a useful tool for predicting inorganic N losses in manure amended soil under furrow irrigation.

Introduction

Maize is one of the most important cereals utilized by a large proportion of the world population (FAO, 1992, Tollenaar and Dwyer, 1999). It is preferably consumed as food, while fodder and raw material are used by agro-based industries (Arun-Kumar et al., 2007, Witt and Pasuquin, 2007). The production of maize is mainly dependent on water and nutrient availability (Badu-Apraku et al., 2011). In many regions of the world maize production is characterized by low yield due to inadequate irrigation and lack of nitrogen (N). Less precipitation and higher water losses in the form of infiltration and evaporation are causing many farmers in different regions of the world to face water shortages (Nelson and Al-Kaisi, 2011). In addition, limited reservoir water supplies have encouraged farmers to search for new methods of crop planting with higher water use efficiency (Payero et al., 2006). On the other hand, N as one of the most mobile soil nutrients is prone to moving beneath the root zone thus reducing maize yields (Iqbal et al., 2010, Khan et al., 2013, Khan, 2008) and decreasing ground water quality (Bagges and Watson, 2000, Fu et al., 2006). As a result substantial research attention has been drawn towards nitrogen and irrigation water management in order to attain sustainable maize yields without negative environmental impacts. Furrow irrigation is a traditional technology widely used for maize production in Pakistan. Its advantages and disadvantages are well known. Among the disadvantages is furrow irrigation’s high water demands (Cartwright et al., 2001). It can cause harmful soil waterlogging when followed by rainfalls. Soil swelling and crack formation between irrigations reduce water application efficiency and make furrow irrigation challenging (Amosson et al., 2001). Despite these known drawbacks furrow irrigation remains a dominant technology for maize production mainly due to its small capital investment requirements.

Furrow irrigation parameters, such as furrow spacing and size are designed according to soil texture characteristics and the crop grown. Typically, a 1:1 (ridge:furrow) aspect ratio with 30 cm, 60 cm and 75–150 cm spacing is recommended for coarse sand, fine sand and clay soils, respectively, while double-ridged furrows are also used (Brouwer et al., 2014). Recent studies have shown that ridge to furrow aspect ratio may affect water use efficiency and crop yields (Li and Gong, 2002, Zhou et al., 2009). Li and Gong (2002) observed 27.9% higher maize yield at a 2:1 (ridge:furrow) aspect ratio compared to a 1:1 ratio as a result of more efficient use of precipitation by plants. Chen et al. (2011) showed experimentally that a 60 cm ridge width provided soil water content and nitrate nitrogen distribution that better met crop requirements in comparison to 30 and 90 cm ridge widths.

Nitrogen fertilizers are usually broadcast in granular form in Pakistan onto the bottom and sides of the furrow, which often results in N losses through volatilization and leaching below the root zone before it can be taken up by plants (Siyal and Siyal, 2013). High cost, potential polluting effects, and higher nutrient losses from chemical fertilizers has led many farmers in Pakistan to use less costly and more environment friendly nutrient sources, including organic wastes from different industries (Iqbal, 2010, Khan, 2008). The use of organic manures as a source of nutrients has been proven beneficial in improving maize yields and soil fertility (Jamwal, 2005). Among different manures, poultry manure is known to be superior in its nutrient contents (Bari, 2003) and has been shown to perform well in sustaining maize yields (Khan, 2008). Press-mud is also a rich source of nutrients and organic matter (Ibrahim et al., 1993, Nisar, 2000) and is produced by Pakistan’s sugar industry (Nadia and Khwaja, 2006) in large amounts (1.2 million tons per year). Therefore land application of press-mud is becoming a common farmer practice in Pakistan (Ghulam et al., 2010, Iqbal et al., 2014). Press-mud mixed with soil has been reported to increase maize yield (Muhammad and Khattak, 2009, Iqbal et al., 2014). In addition to directly supplying nutrients, press-mud may release fixed phosphorous in the soil after application (Marschner, 1995, Mengel and Kirkby, 2001) which deficiency is a great problem of Pakistani soils (NFDC, 2001).

Application of organic fertilizers in Pakistan is on the increase, thus ultimately leading to nitrogen accumulation in soil. The low capacity of many soils to adsorb NO₃⁻ then causes high nitrate losses. This reduces N-fertilizer efficiency and increases the risk of groundwater contamination in this region (Siyal et al., 2012). Results of water quality monitoring in Pakistan showed that among 747 samples collected from a wide range of irrigated and non-irrigated lands approximately 19% have a nitrate nitrogen concentration above the safe limit of 10 mg L⁻¹ (Tahir and Rasheed, 2008). The groundwater contamination level was higher in Balochistan and Punjab compared to other provinces of Pakistan, where nitrate levels were above the limit in 23% of the samples.

Mathematical modeling is a powerful tool for optimizing water and nutrient regimes in soils for crop production. Simulation models with different levels of complexity have been used for decades to predict soil nitrogen transport under a variety of management scenarios. Among commonly used one-dimensional plot- and field-scale models are: Chemical Movement in Layered Soils (CMLS, Nofziger and Hornsby, 1986), Pesticide Root Zone Model (PRZM, Suárez 2005), LEACHN, the nitrate version of LEACHM (Leaching Estimation and Chemistry Model, Wagenet and Hutson, 1989), Nitrate Leaching and Economic Analysis Package (NLEAP, Shaffer et al., 1991, Follett, 1995), Agricultural Production Systems Simulator (McCown et al., 1996), UNSATCHEM (Suarez and Šimůnek, 1997), and the Decision Support System for Agrotechnology Transfer (DSSAT, Hoogenboom et al., 1999), which have been developed to simulate vertical water flow and nitrogen cycle in soils. A limitation of one-dimensional models is their inapplicability to furrow irrigation, where water and chemical fluxes are not unidirectional (i.e., strictly horizontal or vertical). The development of the two- and three-dimensional software HYDRUS-2D (Šimůnek et al., 2011) allowed for the analysis of both vertical and lateral fluxes of water and chemicals from a source with complicated geometrical boundaries. This is particularly important for furrow and drip irrigation, where flux directions change over time due to local temporally and spatially variable gradients in water pressure heads and changing boundary fluxes. HYDRUS computer software packages have been implemented in recent years for modeling nitrogen transport in soils under furrow and drip irrigation (Ajdary et al., 2007, Crevoisier et al., 2008, Ebrahimian et al., 2013, Gärdenäs et al., 2005, Hanson et al., 2006, Khalil, 2008, Mailhol et al., 2007, Ramos et al., 2012, Siyal and Siyal, 2013, Siyal et al., 2012, Tafteh and Sepaskhah, 2012). However, the majority of prior modeling efforts considered only nitrogen from mineral fertilizer sources applied in different forms, including granulated form (Ebrahimian et al., 2013), as ammonium nitrate dissolved in irrigation water (Phogat et al., 2013, Ramos et al., 2012), as urea (Hanson et al., 2006, Tafteh and Sepaskhah, 2012), or as dissolved in soil solution (Crevoisier et al., 2008, Mailhol et al., 2007, Siyal et al., 2012).

Nitrogen is present in both organic and inorganic forms in organic fertilizers such as manure and compost. Such dual presence increases the difficulty in assessing nitrogen losses from organic fertilizers. Even though N losses from composts can cause the same or even larger environmental pollution than mineral fertilizers, their assessment received much less attention. Only a few experimental studies addressed nitrogen losses from organic sources in irrigated systems, while nitrogen losses from poultry manure and press-mud compost application as solids in furrow-irrigated soils remain generally unknown. These losses are difficult to measure directly, because furrow irrigation creates a highly nonuniform water distribution which results in preferential water and nitrogen transport in the soil profile. This is where modeling can be particularly advantageous in this context of experimental non-uniformity and variability, since it allows for the assessment of nitrogen losses from organic sources. To the best of our knowledge no attempts to model nitrogen losses from organic sources in furrow irrigated systems have been reported yet.

This study aims to optimize application rates of organic fertilizers and to optimize the ridge to furrow aspect ratio of furrow irrigation systems so as to meet plant requirements as well as reduce losses of nitrates from the root zone in conventional furrow irrigation systems in Pakistan. Specific study objectives were: (i) to evaluate the effect of application rate for two organic fertilizers on losses of inorganic N from furrow irrigated soil, and (ii) to estimate N losses from furrow irrigation systems with 1:1 and 2:1 ridge to furrow aspect ratio.