The sand fraction was separated from the clay and silt fractions by wet sieving through a 0.05 mm sieve. Water retention at various tensions was determined using a pressure plate. Plant-available water holding capacity was estimated as the volume fraction of water retained between 33 and 1500 kPa. A sample of < 2-mm , air-dry soil was placed on a porous ceramic plate and wetted by capillary action; gravimetric water content was measured following attainment of equilibrium at 33 and 1500 kPa. Soil pH was measured 1:2 in H2O and 1.0 M KCl. Phosphate retention was determined using the method of Blakemore et al. and the Bray-1 extraction was used as an estimate of available P . Exchangeable cations were displaced by 1 M NH4OAc at pH 7.0, then the cations were measured in the supernatant using an atomic absorption spectrometer . The cation exchange capacity was determined in 1 M NH4OAc after extraction of NH4 + by 10% NaCl as a measure of CEC. Base saturation was calculated as the sum of base cations by 1 M NH4OAc divided by CEC. Sulfate-sulfur was extracted using monocalcium phosphate as outlined by Schulte and Eik and available micro-nutrients were determined by DTPA extraction . All weight percent data were reported on an oven-dry basis . Non-sequential selective dissolution in Na-pyrophosphate and ammonium-oxalate was used to characterize Fe, Al and Si in various pedogenic pools. Total C and N concentrations were determined on ground samples by dry combustion using a Costech C/N analyzer . Soil microbial biomass C and N were measured using chloroform fumigation and direct extraction with 0.5 M K2SO4 . Briefly, 10 g oven-dry equivalent samples were fumigated for 48 h in the dark,plastic pot and then C and N were extracted with 0.5 M K2SO4. Similar extraction was applied for non-fumigated samples. The non-fumigated control values were subtracted from fumigated values as an estimate of microbial C and N. A Kec/Ken factor of 0.35 was applied for both C and N . Carbon mineralization was measured in the topsoil and subsoil by incubating duplicate soil samples in the dark under laboratory conditions over a 119-day period.
Soil moisture was adjusted to ∼ 80% of field capacity and pre-incubated for one week prior to starting the long-term incubation. Soils were incubated in sealed Mason jars fitted with septa. Carbon dioxide in the headspace of each soil sample and blanks with no soil was measured each week using an Infrared Gas Analyzer. The CO2 emission was normalized to initial total C content of each soil and expressed as CO2-C mg kg−1 soil C. In addition, net N mineralization was measured on these same samples at the end of the 119-day incubation by determining concentrations of mineral N in 1 M KCl extracts at time zero and at 119 days. Quantification of NO3 – used the vanadium chloride method and NH4 + the Berthelot reaction with a salicylate analog of indophenol blue . A correlation analysis was performed to assess soil properties most strongly affected by land-use changes, using IBM SPSS Statistics 22. 2013.All soils were well drained with an A horizon overlying Bw horizons that extended to the depth of investigation . Soil particle-size distribution was similar among the four sites with the majority of the horizons having a loam texture . Some distinct changes in particle-size distribution within various pedons are attributable to more recent tephra deposition that resulted in burial of the former soil profile. Bulk density in subsoil horizons was very low , characteristic of soils formed in volcanic ash . Db was also low in the A horizon of the pine forest , but was higher under agricultural management due to traffic compaction resulting in a reduced pore volume. The agricultural soils displayed a distinct increase in Db and a reduction in total porosity in the topsoil horizons compared to the pine forest soil. Given the low bulk densities, total porosity was correspondingly high, ranging between 60 and 77%, with values decreasing in surface horizons with agricultural management. Plant-available soil water was generally in a narrow range with the exception of the surface horizons of the pine forest soil . The water retention capacity varied from 37 to 53% in topsoil horizons and from 45 to 51% in subsoil horizons with the lowest values in the pine forest.
Soil pH-H2O increased from very strongly acid in the pine forest and tea plantation to moderately acid in the horticultural crops with fallow and intensive cultivation . Regardless of land use, all soils in this study had low CEC characteristic of acidic Andisols dominated by allophanic materials . The lowest values occurred in the pine forest and the highest values in the horticultural soils. The pHKCl-pHH2O values ranging between −0.1 and −0.5 were indicative of a soil colloidal fraction dominated by variable charge materials . Especially notable is the very low base saturation and concentrations of exchangeable Ca and Mg for the PF and TP soils . Exchangeable base cations are a common limiting factor for horticultural production in the studied Andisols since these nutrient cations are extremely low under pine forest. While the horticultural management practice of applying horse manure and lime did not appreciably increase the measured CEC, it was remarkably effective in increasing exchangeable base cations . For example, exchangeable Ca, Mg and K increased from 1.5, 0.3 and 0.2 cmolc kg−1 in the pine forest to 26.3, 3.5 and 1.0 cmolc kg−1 in the intensive horticultural crops, respectively . The high base saturation of over 100% under horticultural land uses compared to < 23% for the pine forest and tea plantation .Organic C concentration in A horizons was highest in PF and 1.0 to 2.0% lower under agricultural management . In contrast, organic C was lower in the PF subsoil while the agricultural sites had elevated organic C concentrations in several subsoil horizons. Organic C stocks in the upper 100 cm of the soil profile were calculated by summing the organic carbon stocks in each individual horizon were present). Organic carbon stocks followed : TP ≈ IH > FH > PF . The agricultural soils contained more organic carbon than the pine forest soil. While horse manure was added to the IH soil for the past 7 years, the TP and FH soils received no organic matter amendments and still had similar pedon organic matter stocks.
As a direct comparison, the IH soil receiving horse manure contained only slightly more organic C than the FH soil located 4 m away that received no horse manure and was fallowed over the past 7 years. Dissolved organic carbon concentrations were appreciably higher in the PF topsoil and throughout subsoil horizons of the TP profile . The horticultural soils tended to have lower overall DOC concentrations than PF and TP land uses. Total N concentrations followed a similar distribution to organic C concentrations among sites with total N stocks in the upper one meter of soil following : IH > FH ≈ TP > PF . The C:N ratio was lowest in the upper 50 cm of the IH and FH soil profiles , while values for PF, TP and lower soil horizons at all sites were generally in the range 16 to 19. The highest concentrations of inorganic N were found in the IH pedon and were dominated by NO3 – . In contrast to the IH soil dominated by NO3 – , inorganic N concentrations were dominated by NH4 + in the TP, FH and PF soils with the highest value in the TP soil and lowest under FH land use. High P fixation , characteristic of Andisols, was exhibited for all land-use types. Under forest vegetation , the soil P retention was consistent at 97% throughout the entire pedon . Change of land use to TP and FH did not appreciably affect P fixation. However,grow bag the IH land use receiving application of horse manure for the past 7 years showed appreciably lower P fixation in the upper 40 cm. Reflecting the high P fixation, available P content was below the detection limit for all horizons of all land-use types, except for the upper horizons of the IH land use .There were several significant correlations among soil properties . Oxalate-extractable Sio showed a positive correlation with the clay fraction, while Feo had a strong negative correlation with pH and exchangeable Ca and Mg. In contrast, Alo showed no significant correlations with other soil properties. For organo-metal complexes , Alp had highly negative and positive correlations with the clay fraction and organic C, respectively. However, Fep showed no significant correlations with other soil properties. Soil pH showed a highly negative correlation with P retention and Feo, along with a positive correlation with exchangeable cations , total N and Db. Soil bulk density showed a positive correlation with exchangeable cations and negative correlation with P retention. P retention had a negative correlation with exchangeable cations .Andisols are characterized by low Db and high porosity due to the abundance of amorphous and poorly crystalline materials and organic matter that contribute to highly stable and very well structured soils under natural conditions. However, the low natural Db may change due to anthropogenic activities.
The evidence was revealed by soil tillage under intensive horticultural crops contributing to increased Db from compaction by potential destruction of soil aggregates due to physical mixing/abrasion by tillage operations. Tillage was reported to destroy macropore pathways of Andisols in Mexico resulting in a lower in- filtration and permeability of topsoil horizons .Chemically, the exchangeable cations have positive significant correlation with Db, indicating the increase in soil exchangeable cations gave rise to the increased soil bulk density . This is probably due to the role of Ca and Mg ions derived from lime and manure in binding soil particles, resulting in the change of soil friable structure under forest to more compact aggregate formation under intensive horticultural cultivation. The water retention capacity varied from 37 to 53% in topsoil horizons and from 45 to 51% in subsoil horizons with the lowest values in the pine forest . These data indicate that the number of soil pores storing plant-available water is lower in the forest Andisols than those converted for agriculture. In other words, the water retention capacity has increased about 50% following conversion from pine forest to agriculture. This implies that the compaction associated with tillage is responsible for increasing the water retention capacity through conversion of macropores to meso/micropores. The water retention capacity in this study was higher than for cultivated Mexican Andisols reported by Prado et al. . The high water retention in Andisols is caused primarily by their large volume of meso/micropores . Formation of these meso/micropores is greatly enhanced by poorly crystalline materials and soil organic matter . Buytaert et al. studied toposequece of Andisols in south Ecuador and reported the large water storage capacity as revealed by water content ranges from 2.64 g g−1 at saturation, down to 1.24 g g−1 at wilting point. The long-term cultivation of agricultural soils in this study has not caused appreciable degradation to the overall Db, porosity or water retention characteristics of these Andisols. While macroporosity was decreased by tillage, the macropore content of topsoil horizons remained > 15% providing adequate infiltration and soil aeration. The loss of macropores is compensated for by the increase in meso/micropores that contribute to increased plant-available water holding capacity. In spite of the increase of bulk density and loss of macropore capacity, field observations confirmed that the agricultural soils in this study retained their high infiltration capacity with no evidence of surface runoff. In Italy, well developed Andisols on flow-like landslides over 70 years experienced low run off and minimal soil erosion owing to a good infiltration in spite of the high slope steepness and the anthropic pressure associated with land management .The pine forest soil was very strongly acidic owing to the strong leaching regime associated with the isothermic/perudic climatic regime. Applications of lime and more recently horse manure to the IH soil were effective in raising the pH of the horticultural soils . In spite of the low soil pH values in the tea plantation, the potential for Al3+ toxicity was not evident as ascribed to the low exchangeable Al3+ concentrations . Threshold values for Al toxicity are generally considered about 2 cmolc kg−1 for common agricultural crops and 1 cmolc kg−1 for Al-sensitive crops .