Pig compost, known as “biostabilized” in Chile, is derived from the pork industry; Chile produced approximately 300,000 Mg in 2008. This product is characterized by its high Cu and Zn concentrations which do not comply with the limits indicated by the Chilean Compost Regulation [1] . Despite the fact that this organic amendment is an important nutritional source for agricultural crops, it has a comparatively lower cost than conventional fertilizers [2] [3] just like amendments derived from other types of meat production [4] - [6] . In contrast inorganic fertilizers, the organic amendments have high organic matter content and can be used as source of slow-release nitrogen. Therefore, agricultural production systems that use these amendments contribute to improving soil physical, chemical, and biological properties [4] [7] - [9] . However, the net supply rate of certain nutrients, such as nitrogen (N), is lower than the rate produced by conventional fertilizers [6] [10] [11] . In many cases, this situation has led farmers to increase the applied amendment rate to compensate N in the crop; this fact could cause certain environmental risks. The increment in the concentration of other elements generates potential health risk since they can enter the food chain or could have a negative effect on the plant development [12] - [14] . Heavy metals, such as Cu and Zn, are two of these elements; their high rates in the PC can affect crop yield through direct effects on the plant or indirect effects derived from soil nutrient antagonism [12] [15] - [17] . In this respect, several works about the effect of organic amendments on heavy metals in plant and soil had been reported in the world [12] [15] - [18] . Moreover, for the Chilean conditions, there is not information published.

Therefore, in order to contribute to a better knowledge of using PC and its effects on a very important crop for world food supply, as well as for Chilean agriculture, the objective of this study was to evaluate the use of increasing PC rates on corn production, Zn and Cu concentrations in plant tissues, and the extractable concentration of these elements in the soil.

A positive DM production response was observed with the increasing PC rates (Figure 1), which indicates that

Different letters in the same column indicate a statistical difference according to Tukey’s test (p < 0.05); C: control without fertilization; CF: conventional fertilization at 350 kg N ha⁻¹ rate; PC1: pig compost at 350 kg N ha⁻¹ rate (15.33 Mg PC ha⁻¹); PC2: pig compost at 700 kg N ha⁻¹ rate (30.65 Mg PC ha⁻¹); PC3: pig compost at 1400 kg N ha⁻¹ rate (61.31 Mg PC ha⁻¹) CV: coefficient of variation.

Different letters in the same column indicate a statistical difference according to Tukey’s test (p < 0.05); C: control without fertilization; CF: conventional fertilization at 350 kg N ha⁻¹ rate; PC1: pig compost at 350 kg N ha⁻¹ rate (15.33 Mg PC ha⁻¹); PC2: pig compost at 700 kg N ha⁻¹ rate (30.65 Mg PC ha⁻¹); PC3: pig compost at 1400 kg N ha⁻¹ rate (61.31 Mg PC ha⁻¹); CV: coefficient of variation.

there was no negative effect of using PC in corn for the evaluated rates, associate to its slow N mineralization [2] [6] [10] . Dry matter production in the fertilized treatments fluctuated between 20.9 and 30.6 Mg・ha⁻¹; these values were similar to those mentioned by [8] for silage corn in the same production area. No difference in DM production was observed in Talca (Figure 1) for the evaluated treatments (p > 0.05); this is attributable to soil chemical properties (Table 2). Dry matter production obtained in the fertilized treatments fluctuated between 31.8 and 35.0 Mg・ha⁻¹, which was similar to values indicated by [26] .

Corn plant Zn concentration values obtained in Chillán were higher than those found by [28] for different corn plant tissues amended with sewage sludge compost; while the values obtained in Talca were lower than those found by those authors. The quantitative differences between both locations is likely due to the initial chemical properties of each soil (Table 2) since the availability of each nutrient is affected by the concentration of other nutrients, such as phosphorus and calcium, and the presence of secondary crystalline minerals found in higher proportions in Talca [22] . In general, an inversely proportional relationship was observed between corn plant Zn concentration and increasing PC rates in both locations (Figure 2). In contrast, [18] indicate significant increases in Zn concentration in ryegrass (Lolium multiflorum Lam.) stems and roots amended with sewage sludge at a rate of 140 Mg・ha⁻¹ (730.5 mg・Zn・kg⁻¹) as compared to C and CF with NPK contributions equivalent to using organic amendment.

For the corn plant Cu concentration in quantitative terms, an inversely proportional effect was observed for both locations with the increasing PC rates (Figure 3), associate to the reactions with the organic compounds applied [22] . Copper concentration fluctuated between 2.0 and 3.4 mg・kg⁻¹ in Chillán and between 0.6 and 4.5 mg・kg⁻¹ in Talca; these were below the normal range mentioned by [27] for corn leaves and similar to values pointed out by [28] for different corn plant tissues amended with sewage sludge compost. [18] experimented with ryegrass amended with sewage sludge at a rate of 140 Mg・ha⁻¹ (204.5 mg・Zn・kg⁻¹) and indicated that applying this organic amendment produced significant increases in Cu concentration in ryegrass stems and roots as compared to C and CF.

The same as in DM production, a positive response in GY was observed with increasing PC rates (Figure 4); there was no negative effect of using PC in corn crop for the evaluated rates, associate to slow availability of the total N applied and the chemicals reactions between organic compounds and the nutrients applied with the PC [29] . In Chillán GY in the treatments applied fluctuated between 14.4 and 17.7 Mg・ha⁻¹, which is similar to values mentioned by [30] in central Chile and slightly lower than those found by [26] in south central Chile.

For Talca the GY in the fertilized treatments fluctuated between 18.2 and 20.1 Mg・ha⁻¹; these values were similar to those indicated by [26] and higher than those found by [30] .

Grain Zn concentration values (Figure 5) were generally similar to those pointed out by [28] for corn amended with sewage sludge compost. Unlike the difference obtained in whole plant Zn concentration, there were no quantitative differences in grain concentration between locations; the mean value in the treatments with PC were 30.0 and 32.3 mg・kg⁻¹ in Chillán and Talca, respectively.

The mean grain Cu concentration was higher in Talca than Chillán (Figure 6); this fact may be was attributable to the different chemical properties of both soils (Table 2). Grain Cu concentrations in both locations were similar to those mentioned by [28] for corn amended with sewage sludge compost.

Respect to the soil in both locations, the lower pH obtained with the conventional fertilization (urea) use, which is in agreement from this reported by [2] who indicated that the application of conventional fertilizer (urea) produced soil acidification. The acidification generated with the PC use in high rate for the first stratum in Talca is related to the use of composted organic amendments and its carbon mineralization and release of organic acids, as has also been reported by several authors [2] [11] [13] [29] [31] and too can be attributed to the nitrification of ammonia-N [22] [29] .

Although a difference in pH existed at the start of the experiment (Table 2 and Table 4), this difference increased in Talca. This is due to the higher level of clay content and its structural characteristics (crystalline clay type 1:1 and 2:1 soils in Talca compared to the amorphous clay soils in Chillán), which produced base accumulation by illuviation in the second soil stratum [22] .

Soil Zn concentration in Chillán showed the same effect in both soil depths and the highest concentration was obtained with PC3 (1400 kg N ha⁻¹), which was only significantly different under C and CF treatments and no significant differences were seen among PC treatments (Table 5). The high variation coefficient found for this nutrient in both soil layers suggests, that long-term studies are needed in order to detect soil Zn concentration changes, such as the one carried out by [17] where organic amendments were applied to evaluate the over time, changes in soil heavy metal contents. The highest Zn concentration was found in the first stratum when comparing both soil layers, which was associated with the strong bonding of this nutrient to the organic matter, plus the higher pH in the subsurface soil layer [22] . Similar effects associated with applying increasing rates of sewage sludge compost were reported by [28] . Almost the same Chillán soil, the Zn concentration in Talca soil for first soil stratum are agree with the results obtained by [28] . The mean value of PC treatments for the first soil layer showed a higher Zn content in Talca soil than in Chillan soil, while the higher content in the second soil stratum was found in Chillán than in Talca soil (Table 5). The highest mean value of extractable soil Zn was found in Talca (Table 5), which agrees with the initial concentration of this nutrient in this soil (Table 2). The highest increase in Zn concentration in both soils was obtained in Chillán (Table 2 and Table 5); this is attributable to the type of clay, as well as lower soil pH [22] . The pH effect, which showed differences only between PC3 and C in the first soil layer could have contributed in increasing Zn concentration in this stratum [22] (Table 4 and Table 5). No toxicity reports exist for Zn in agricultural crops in Chile; this slight increase in concentration associated with PC at rates that meet crop nutritional needs does not constitute a risk as this alternative fertilization source.

The highest soil Cu concentration in Chillán was generally obtained in the first stratum (Table 6) associated with the low movement of this nutrient in the soil [22] . The results obtained agree with those registered by [28] in an increasing rates of sewage sludge compost study as well as differences in concentration between soil depths. As well as in Talca soil there were not differences between treatments for the soil Cu concentration, this lack of response may be due to the high coefficient of variation obtained in both soil strata (Table 6). Similarly to Chillán, a higher Cu concentration was found in the first soil stratum, but extractable soil Cu concentration in both stratums was higher in Talca (Table 6). However, considering the average applied treatments, the extractable soil Cu content regarding to the initial value, increased only in the Chillán soil (Table 2 and Table 6). This fact was attributable to the lower initial concentration and type of clay as well as lower soil pH [22] . The lack of differences among treatments and the slight increase in the extractable Cu concentration with regard to the initial value is an advantage for this organic amendment which has high Cu concentration. Soils high in crystalline clays, such as Talca soil are associated with the lack of response to Cu and Zn applied with the PC treatments. In addition, the soil pH in Talca did not affected the Cu availability, since only PC3 produced a lower pH than C in the first soil layer (Table 4), therefore the extractable soil Cu contents in this location and depths did not show significantly differences among treatments (Table 6). This effect is associated to chemicals reactions with the organic compounds presents in PC [29] .

The results obtained indicated that the composted pig manure applied did not negatively affect corn crop cultivated in two soils with different pedological characteristics. In both soils, applying PC at rates adjusted to the N nutritional needs for corn production, showed a slightly, non-significant decrease of GY production without effect on grain Zn and Cu contents with respect to the conventional fertilization. At the same time, the DM production and Zn concentration in plant were not affected with this PC rate with respect to the conventional fertilization, but the Cu concentration in plant was decreased significantly in one of the two locations. An increase with increasing application rate in extractable soil Zn and Cu contents was observed in both soils, where the greater impact was obtained on the superficial soil stratum. However, both elements remained far below levels considered phytotoxic. Nevertheless, further medium and long-term monitoring should be undertaken in order to evaluate the possible changes in soil of these trace elements.

The author is thankful to the Servicios Pucalán Sur Company for co-financing this experiment and to the SEMAMERIS Company for facilitating their installations in their Talca experimental station.