**4. Discussion**

In the first and second year after reconversion from the pastures, the ratio of the soybean yield in agropastoral plots to that in control plots in Exp.-1 was higher than that in Exp.-2. Therefore, it was thought that the positive effects on soybean productivity of the agropastoral system with extensive grazing and long term pasture was higher than that with intensive grazing and short term pasture. Macedo et al. (2004) reported that the mean ratio of the soybean yield in the first year between all 4 years agropastoral plots and control plots was 1.12 (calculated from their table) under a conventional grazing system which the weight gain of cattle per hectare was one-third that of our intensive grazing system. But, it had higher grazing pressure than that of our extensive grazing system. Therefore, it was considered that the positive effect on soybean productivity was large as the grazing pressure was low.

However, it was thought that it would be lost in about four years even if agropastoral system with the extensive grazing and long term pasture (seven years) was conducted. So, it was important for this system to convert a field into a pasture continually.

In the drought year, the positive effect on soybean productivity was clear. It was considered as a reason that the phosphate accumulation at soil surface was dissolved. Many studies reported that the root of soybean was distributed within a shallow soil layer with no-tillage system (Iijima et al., 2007; Izumi et al., 2009). In addition, the phosphate accumulates near the soil surface, which restricts a crop root distribution within a shallow soil layer with a notillage system (Holanda et al., 1998; Seki et al., 2001). Plants with shallower root systems have a disadvantage for uptake and sensitive to drought (Schwinning ,1988).

In general, soybean does not grow well in acidic soil, and a pH range of 6.0 to 6.5 is best for soybean cultivation (Kokubun, 2002). In our system, soil pH in top soil improved from 5.89 to 6.11 over 3 years in the pasture, and conversely, soil pH became lower and the soil acidified in the control plots. Therefore, possibly the improvement of soil pH had same effect on the increase of the soybean yield.

Studies have reported that the accumulation of organic matter in soil is promoted by introducing agropastoral systems (Miranda et al., 2004; Salton & Lamas, 2007; Shimoda et al., 2010). In general, organic matter develops the soil aggregate structure and improves the water-holding capacity (Uwasawa, 2002). However, the accumulation of organic matter was promoted in Exp.-1 and not promoted in Exp.-2. Ogawa & Mitamura (1982) also reported that the accumulation of organic matter was not promoted by grazing. It was a reason that the root growth was inhibited by cutting the aboveground part of the grass (Davidson & Milthorpe, 1966a; 1966b). In addition, a lot of grass was grazed and much cattle meat (1.54 ton/ha) was carried out from the pasture every year under our intensive grazing. Therefore, it was thought that the positive effects on soybean productivity in Exp.-1 was larger than that in Exp.-2 by the promotion of organic matter accumulation.

In Exp.-1, at soil surface (0-10 cm at depth) in agro-pastoral plots, the percentage of gaseous was lower and bulk density was higher. Soil compaction of soil surface inhibits soybean production (Ae, 1997). However, since soil sampling was carried out immediately after killing off Guinea grass by herbicide, soil compaction at the surface would disappear rapidly by decomposition of the root of Guinea grass after that. And, since the percentage of gaseous phase of soil in agropastoral plots was higher in the soil layer from 10 cm to 50 cm at depth, it was thought that soil compaction occurred in no-tillage cultivation had improved. In addition, percentage of large aggregate of soil of agro-pastoral plots was higher than that of control plots in each depth. Higher percentage of large aggregate may promote inflow of air to underground. Inflow of air to underground promotes nitrogen fixation of soybean (Ae 1997). Therefore, it seemed that the improvement of physical properties of soil has contributed to recovery of soybean productivity.
