Management of Agroforestry for Soil Improvement: Comparison
Please note this is a comparison between Version 1 by akash chichaghare and Version 2 by Catherine Yang.

Agroforestry integrates woody perennials with arable crops, livestock, or fodder in the same piece of land, promoting the more efficient utilization of resources as compared to monocropping via the structural and functional diversification of components. This integration of trees provides various soil-related ecological services such as fertility enhancements and improvements in soil physical, biological, and chemical properties, along with food, wood, and fodder. 

  • agroforestry
  • soil health
  • land degradation
  • microclimate
  • soil fertility

1. Species Selection and Density

Trees trap more sand dust than shrubs due to their larger canopies and dense foliage. As a result, the soil beneath trees is likely to be more nutrient-dense than that beneath shrubs. Thus, trees should be favored over shrubs for soil improvement. Augustine et al. [1][22] suggested that an increase in SOM in agroforestry systems after three years of planting Gliricidia improved the soil’s nutrients and its ability to support agriculture, but not enough for sustainable production, as this technology alone did not offer enough soil cover to decrease erosion and should be used in conjunction with further management initiatives to lower erosion rates. Additionally, farm management operations such as thinning, trimming, and mulching should be carried out [2][44]. Wartenberg et al. [3][23] reported that a rise in tree diversity in complex agroforestry significantly boosted the soil organic carbon at the topsoil but had little effect on the deeper soil layers. According to Eddy and Yang [4][2], there were not many more advantages to agroforestry for soil health over monoculture, even when the crop diversification is just doubled. The selection of crop species with niche complementarity roles may be more important to agroforestry than species selection advantages from broad crop diversification. Combining species with various root depths, integrating shade-tolerant species beneath the crowns of the foundational trees, and including nitrogen-fixing species into the mixtures to enhance soil nitrogen provision improves the resource-use efficiency of the system [4][2]. The selection of species with various root structures may facilitate the partitioning of the soil water intake from different soil levels, resulting in less competition and less water loss via deep percolation. Douglas et al. [5][48] reported 11–18% greater total soil carbon mass rates in open pastures as compared to pastures with poplar systems, whereas pastures with alder systems were 2–6% greater as compared to open pastures, suggesting a choice of proper and suitable species based on the locality and climate for soil improvement. A study by Diallo et al. [6][30] indicated that under both F. Albida and P. reticulatum, the soil Na, Ca, P, Mg, P, K, NH4-N, OC, and pH levels were considerably higher as compared to other tree species and open areas, suggesting that F. albida and P. reticulatum are more appropriate trees for planting in FMNR (farmer-managed natural regeneration) agroforestry parklands for the improvement of soil fertility, food, and fodder production than any other shrubs or trees. The role of trees in agroforestry systems is determined by a variety of elements, including the species (rooting depth), size and spacing, soil type, rainfall volume and pattern, and dry season severity [7][53]. C. africana’s water intake was mostly centered in the top 90 cm but extended down to 130 cm deep during dry seasons. This indicates that the first 40 cm coincided with the active root zone of the coffee [2][44]. Still, Sarmiento-Soler et al. [2][44] did not find any water competitiveness between coffee and banana or coffee and C. africana, since the coffee plants’ water usage remained static throughout the systems. Bisht et al. [8][36] compared a wheat–poplar agroforestry system and sole wheat cropping system in Uttarakhand, India, and proved the role of the agroforestry system in the improvement of soil health against climate-related extremities. In the wheat–poplar agroforestry system, the highest pH, EC, available N, and K levels were observed with the UP-2572 wheat variety, while the highest SOC and available P levels were observed with DBW-711. Another study carried out in semi-arid region of Northwest India by Sirohi and Bangarwa [9][33] reported that higher available soil N, P, and K levels were observed in a 5 m × 4 m geometry than in 10 m × 2 m and 18 m × 2 m × 2 m geometries (paired row) for 7–8-year poplar-based intercropping. Thus, it was recommended that poplar trees be planted at a spacing of 5 m × 4 m as the most appropriate way to improve the soil fertility via the accumulation of leaf litter in the semi-arid and arid areas of Northwest India. Akpalu et al. [10][29] found that the existing rates of 1.09 and 2.29 trees of F. albida per ha in parklands in the Sudan Savannah zones and Guinea, respectively, were insufficient for exploring the full potential of F. albida in terms of soil fertility improvements. Thus, there is a need to increase the density on parklands in the Sudan Savannah zones and Guinea to 59 and 37 trees per ha, respectively, to fully achieve fertility improvements among resource-poor farmers, which could add about 100, 3.45, 4.63, and 1698.37 kg ha−1 of N, P, K, and OC, respectively, to the soil per year. However, the study by Wu et al. [11][51] in China warned that in rubber agroforestry, the dense planting of herb species should be avoided, since an increase in the species composition can negatively influence the soil moisture owing to increases in root pores and organic matter, increasing the infiltration and resulting in increased leaching. Such complex, climate-smart, and productive agroforestry systems need robust site- and species-specific knowledge in order to increase their climate resilience [12][54]. When wheat intercropped with deciduous poplar tree rows was arranged in the north–south and east–west directions in Haryana, higher tree diameter and height growth values were reported in the north–south-oriented tree rows [13][42]. The windbreak’s orientation and the tree spacing can enhance the system’s microclimate and ecosystem services, leading to increased production and financial gains in the semi-arid areas of India [13][42].

2. Nutrient Management and Fertilizer Application

In agroforestry, the judicious use of appropriate combinations of organic and inorganic fertilizers promotes soil mineralization and N availability. Kumar et al. [14][52] reported that the application of FYM @ 10 Mg ha–1 + a recommended dose of chemical fertilizer (NPK) effectively stimulates C mineralization in Terminalia chebula-based agroforest in the foothills of the Himalayas in India. Thus, the integrated use of organic and inorganic fertilizers should be encouraged in order to improve the C mineralization and inorganic N pools, which can lead to enhanced nutrient availability to plants and higher crop productivity [14][52]. Another study by Kannur et al. [15][55] found that the integrated application of FYM, Azotobacter, and PSB on Capsicum frutescens under a 2-year-old rubber plantation proved to be better in terms of improving the physical and chemical soil properties under an agroforestry system. Akpalu et al. [10][29] recommended that it would be financially reasonable to combine an inorganic P source with the organic material in the management of F. albida parklands in Ghana because the tree leaf biomass typically contains a higher N/P ratio than that required by the crops, while the P may become deficient in an attempt to supply N via F. albida leaf litter application. Manson et al. [16][56] found lower pH levels in coffee farms using agrochemicals and farms dominated by Eucalyptus trees in West Java. Thus, planting native fruit tree species rather than eucalyptus trees as shade trees for coffee plants with the application of organic manure or liming is recommended.
Although higher SOM levels, soil C stocks, total C pools, and total N levels were reported under the coffee–banana system than banana sole cropping in central Uganda, precautions to avoid P depletion should be taken, as under both farming systems the available P levels were limited [17][57]. Zake et al. [17][57] suggested the application of well-composted manure @ 20 Mg ha–1 per year to solve the soil P limitations. The integration of livestock into their farming systems might be beneficial. The age of the trees, the crown morphology, the phytochemical composition of the litter and its nutrient content, and the root turnover rate determine the improvements in soil properties. Because of the low tannin contents in their leaves, F. albida and P. Reticulatum decompose more quickly and release more soil nutrients [18][58]. The potential for agroforestry to restore the land via ‘internal restoration’ may depend on local circumstances; thus, the proof is inconsistent and inconclusive [19][59].
The agroforestry ecosystem’s soil nutrients would be better managed if the soil mycorrhizae and soil P were fully taken into account [20][60]. Negative soil conditions, disease concerns, and increased vulnerability to climatic extremes may all lead to agroforestry systems being the best sustainable alternatives.
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