Trends in the nutrient budgets
N budgets for farms 1, 2, and 3 varied significantly among the budgeting approaches. The estimated total N inputs to the farms 1, 2, and 3 using the GNB approaches were approximately 11,110.0, 28,617.9, and 23,845.4 kg/yr, respectively. While estimated using the SSB approach, the total N inputs were 5,065.2, 14,333.4, and 7,473.4 kg/yr for the farms 1, 2, and 3. As expected, anthropogenic N inputs through livestock manure/compost were the largest source of N in the farms in all cases. Other sources including chemical fertilizers, planting materials, BNF and atmospheric deposition added only a small portion to the inputs. The total N outputs calculated using the current GNB approach were approximately 26.4, 88.6, and 68.4 kg/yr, the ideal GNB approach were 67.3, 146.1, and 21.9 kg/yr and the SSB approach were 3,943.9, 11,157.6, and 5,831.8 kg/yr for farms 1, 2 and 3, respectively. The SSB approach showed that hydrologic export (leaching and runoff) and atmospheric loss (volatilization and denitrification) were the two dominant pathways of N outputs, accounting for approximately 53.2% and 45.9% of the total N outputs (average), respectively. Crop harvesting accounted for only a small portion of N outputs (approximately 0.7%).
Similarly, a fairly large difference in the P budget using the GNB approaches and SSB approach was observed. Based on the GNB approaches, the total P inputs to farms 1, 2, and 3 were approximately 1,963.5, 5,031.0, and 4,207.8 kg/yr, respectively and SSB approach estimated the P inputs as 81.5, 1,212.9, and 1,030.0 kg/yr, respectively. P inputs from livestock manure/compost were the dominant source of nutrient in the farms. Other sources such as chemical fertilizers, planting materials and atmospheric deposition accounted for a relatively small amount of the total inputs. The total P outputs for farms 1, 2, and 3 using the current GNB approach were approximately 5.9, 19.2, and 14.9 kg/yr, respectively; the ideal GNB approach were −34.9, −27.3, and 219.3 kg/yr, respectively and the SSB approach were about 40.7, 537.1, and 454.7 kg/yr, respectively. The negative outputs found in the ideal GNB for the farms 1 and 2 implies that P removal from the soil P stocks. The SSB approach identified that P export in leaching was the major pathway of P outputs, accounting for about 76.7% (average) of the total P outputs. Of the total P outputs, on an average approximately 16.1% was lost due to runoff. Crop harvesting was accounted for 7.1% (average) of the total P outputs.
As expected, all the investigated farms showed nutrient surpluses for N and P. Nutrient surpluses in the farms are mainly attributed to livestock manure/compost application. Due to differences in system boundaries, a large variation in nutrient surpluses was observed among the budgeting approaches. In both GNB (current and ideal) approaches, nutrient inputs from livestock manure are considered, whereas as mentioned earlier livestock manure is composted before land application in Korea. Therefore, calculated nutrient budgets based on the GNB approaches ultimately showed large nutrient surpluses as the nutrient loss during composting were not taken under consideration while calculating the nutrient inputs. Moreover, the current GNB approach is unable to reflect the effects of different on-farm manure treatment methods on compost production and does not consider the nutrient loss pathways. In case of the ideal GNB approach, it can be used as a comprehensive agri-environmental indicator as it considers the aGNS to reflect the N loss and shows the potential risk of nutrient surplus to the whole agricultural environment (soil, air, and water). However, the system boundary in the ideal GNB is quite wide and hence requires many data for adequate calculation. Due to lack of data, its contribution to uncertainty is very high and thus difficult to use in managing the nutrients. Furthermore, in the ideal GNB approach, hGNS is not estimated directly rather calculated by deducting aGNS from the gross nutrient surplus. Depending on the manure treatment process, the aGNS varies so as hGNS. The ideal GNB approach therefore cannot depict real nutrient loss. Moreover, the manure management system in Korea is not only limited to solid-liquid separation and composting, but it also includes efforts towards reducing the odor using absorption tower, biofilter, biocurtain etc. So, not all the N lost through denitrification and volatilization are not entering the atmosphere, a portion of N is captured by the odor reduction facilities. So, there is always a chance of overestimating the aGNS in the ideal GNB approach. Despite a detailed approach of calculating nutrient balance, the ideal GNB approach therefore cannot depict the real N loss. The same is true for P also. Although unlike N, P cannot be lost during the manure treatment and composting atmospherically; rather it reacts with other cations and precipitates as struvite. Struvite either deposits on the digester or form scales in the pipes during the closed manure treatment system. In both GNB approaches, P removal through struvite precipitation and loss due to scale formation are not considered. Whereas the SSB approach focuses only on the soil environment. It is a nutrient budgeting approach providing sufficient information on nutrient inputs and outputs, nutrient recycling process and dominant nutrient loss pathways. The SSB approach considers the amount of nutrient applied to the soil after manure treatment as solid and liquid compost. Thereby the difference in the nutrient inputs by the different manure recycling and treatment processes method can be considered directly. Although the nutrient loss to the atmosphere before the application is not considered in the SSB approach, the dominant nutrient loss pathways from after application can easily be identified. The SSB approach also shows the partitioning among different nutrient loss pathways and storage or depletion of nutrients within the soil system. From the above discussion, it can be said that, although the ideal GNB is a detailed nutrient budgeting approach depicting the impact of nutrients produced in the mixed crop swine farms on overall agricultural environment, the SSB approach appears to be a reasonable budgeting approach suitable for the mixed crop-swine farms in Korea and more appropriate for the farmers for efficient nutrient management.
Effect of manure treatment methods on nutrient loading and budgets
Nutrient inputs in agroecosystems above requirements result in nutrient surplus and are one of the main causes of eutrophication in freshwater ecosystems. Intensive livestock farming systems without appropriate manure recycling processes and treatment methods generate large nutrient surpluses [
22]. In this study, the SSB approach revealed that on an average approximately 22.1% of applied N and 53.9% of the applied P was retained in the farms as surpluses and compost was the dominant source of nutrient inputs. Hence, we quantified and compared N and P produced from swine manure through different recycling processes and treatment methods using nutrient loading fluxes reported by Won et al [
6] to suggest an effective on-farm treatment method for reduction of nutrient loading to soil (
Table 8).
For liquid composting, changing to the aeration method from intermittent to continuous reduced the N and P loading about 50% and 47%, respectively. These findings are supported by previous studies [
23]. Factors such as influent characteristics, pH, molar ratio of NH
4, PO
4, and Mg, aeration and temperature control nutrient removal from swine wastewater [
24]. Among the above-mentioned factors, aeration plays a significant role in nutrient removal. Continuous aeration influences the wastewater pH by CO
2 stripping and thus improves the removal efficiency of nutrients [
25]. As CA during liquid composting increases pH, struvite can be formed in alkaline conditions. Struvite produced in the digester either precipitates or forms scale in the piping system as described above. Such struvite cannot be applied in the arable lands but plays an important role in removing nutrients from the liquid manure. Based on the removal efficiency, the CA method during liquid composting is suggested.
Turning (T) only and TA, are the two methods used by the swine farms in Korea for solid composting.
Table 5 shows that change in treatment method from T to TA improved the N removal efficiency by 30.5%, while P removal efficiency decreased by 2.1%. A similar phenomenon was reported by Zhang and He [
26]. Due to microbial nitrification and atmospheric loss through denitrification and ammonia volatilization, N concentration decreased in the solid compost using the TA method [
27]. Unlike N, P is less mobile in the environment and less susceptible to loss during the composting process. Therefore, the TA method of solid composting resulted in decreased N loading, while P loading remained almost the same. Considering the decrease in N loading we therefore support the use of the TA method of solid composting over the T method.
Furthermore, the impacts of suggested treatment methods on nutrient budgets were evaluated by developing soil system N and P budgets for the study farms (CA for liquid composting and TA for solid composting) (
Tables 9,
10). The results showed that changing swine manure treatment methods significantly reduced the farm nutrient surpluses. For farm 1, change in manure treatment method resulted in 49.4% and 47.6% reduction in N and P surplus, respectively. While for farm 2, 44.6% and 7.3% reduction in surplus was observed for N and P, respectively. For farm 3, although the N surplus decreased by 7.4%, the P surplus increased by 1.9%.
Management of surplus nutrients
Effective management of surplus nutrients while achieving agro-economic sustainability and crop productivity poses many challenges. Due to rapid industrialization and urbanization, arable land in Korea is decreasing. Like other countries, farmers in Korea are therefore shifting towards high fertilizer (chemical and compost) input-based farming systems to ensure productivity. Farmers generally think that the more fertilizer they use, higher yield and profit will result. Intensive agricultural practices in Korea started during the mid-1980s and increased until the mid-1990s. Due to the development of national soil fertility database, decision-support system for farmers and promotion of eco-friendly agricultural policies, FA decreased from 689,901 metric tons in 2002 to 450,453 metric tons in 2016 [
5]. However, the numbers are still higher than the most OCED member states. Recent N and P budget studies reported that FA rate for highland crops in Korea is more than 1.5 and 4.5 times, respectively higher than the recommended rate [
8,
9]. Such activities resulted in the retention of surplus nutrients on arable land. Hence, while managing the surplus nutrients along with the environmental aspects, the social and economic perspective should be considered. The fertilizer (chemical and compost) application rate to the crop fields often determined based on crop N requirement [
28]. As a result, P may be applied above the crop P demand and retained in the soil. The retained P can be released in the future due to hydrologic, geologic, climatic events or changes in land management practices and can amplify P related environmental problems.
Moreover, the buildup of large nutrient surpluses from nutrients in animal feeds is another major issue in countries with intensive livestock production like Korea. Goulding et al [
29] reported that low conversion efficiency of nutrients in animal feeds lead to loss of nutrient through excretion. For efficient nutrient management in Korea, swine farms either need to treat the manure using on-farm treatment facilities or send the wastewater to the centralized treatment plants before discharge into water bodies [
30]. The operational cost of the farms having on-farm manure treatment processes is higher than the farms having a simple storage facility. Considering the reduction in nutrient content, post-management cost, social cost and environmental benefits, composting is an effective way to treat the manure. The farms already having composting facilities required no further investment in infrastructure development except installing the aeration system for solid composting. However, operational cost of liquid and solid composting by CA and TA, respectively might be higher than IA and turning. Acknowledging the environmental sustainability and long-term benefits, the Korean government could develop policy and provide subsidies to encourage the farm owners to change the on-farm treatment facilities. Therefore, improving on-farm manure treatment methods along with additional policies (tested and successful in other parts of the world) such as nutrient recovery from wastewater, reutilizing recovered nutrients as fertilizer and feed additives, extensive and organic farming systems, cultivating high yielding varieties for efficient use nutrients, precision farming, distributing compost in nearby specialized croplands, restricting maximum application limit and time, incentives for achieving balance and comprehensive nutrient management plans could be useful in reducing the nutrient surpluses in mixed crop-swine farms [
29].