 This analyzer determines the total amount of N and C in all forms using a flash combustion system joined to an infrared detector and to a thermal conductivity detection system (AOAC International, 1997).
Soil bulk density was determined for all sites and treatments over the three growing seasons using the core technique (Grossman and Reinsch, 2002) with a core sampler with a 7.6 cm diameter core. In the potato trials, four initial composite samples per community were collected prior to planting or application of treatments and three samples in each experimental plot prior to harvesting. In the quinoa trials, three samples per plot were collected prior to harvest. The field-moist soil collected from the cores was weighed, dried at 110ºC for 48 hours, and then reweighed for determination of the total dry weight of the soil in the cores.
The cow and sheep manure, household made compost and the commercial biofertilizer used in all potato trials were also analyzed for selected chemical characteristics. One replicate sample of the sheep and cow manure used in the potato plots in each community, and one sample of the household compost and one of the biofertilizer were collected, and properly labeled in plastic bags. Organic amendments were thereafter air-dried, ground with mortar and pestle and sieved using a sieve with 1 mm openings. All the samples were then analyzed for total organic C, total N, total P and total K. Chemical analyses for total P and total K were determined using standard methods for the University of Missouri Soil and Plant Testing Laboratory (Nathan et al., 2006). Total organic C and N were determined by combustion using a TruSpec® CN analyzer (LECO Corp., St. Joseph, MI).

Soil C and N Mineralization Potential
To determine soil C and N mineralization potential with addition of organic amendments used in the field experiment, an aerobic leaching incubation experiment (Motavalli et al., 1995) was conducted in the laboratory. Bulk sandy loam soil was collected from a depth of 20 cm from a representative farm field in the Vinto Coopani community for use in the experiment. The farm field was in its first cropping year after being in fallow. The soil was air-dried, ground and passed through a sieve with 2-mm openings. Analysis of this soil showed that it contained an average (± std. dev.) of 1.0±0.1% total organic C and 0.09±0.01% total N (Table 3).
Treatments included in the laboratory incubation consisted of rates of dried and ground cow and/or sheep manure, compost and Biofert collected from the field trials. The organic amendments were mixed with the soil at equivalent rates of 7.5, 15 and 30 Mg ha -1 (dry weight basis). To reach the desired rates, 2.82, 5.64 and 11.28 g of organic amendments were added for each kg of soil, respectively. The Biofert was added at an equivalent rate of 0.2 Mg ha -1 in a combined treatment with the sheep and cow manures. For each treatment, 100 g of dry bulk soil was well-mixed with each organic fertilizer rate before the initiation of the incubation. The treated soils were placed in 150 mL Corning filter units and leached initially with 100 mL and subsequently with 50 mL of minus-N nutrient solution at -47 kPa soil moisture tension (Motavalli et al., 1995). Each filter unit was fitted with a 0.22 µm cellulose acetate membrane filter covered with a glass microfiber prefilter of 47 mm diameter with high wet strength and loading capacity. A glass fiber filter of 70 mm diameter, with similar characteristics as the 47 mm diameter filter, was placed on the top of the soil in the unit to prevent dispersion. The filter units were maintained at a constant temperature of 25ºC and incubated for a total of 84 days. Leaching and collection of the leachates for determination of mineralized N (NH 4 + -N and NO 3 --N) were performed after 1, 3, 7, 14, 21, 28, 42, 56, 72 and 84 days of incubation. It was assumed that the initial leaching of the treated soil removed any initial inorganic N contained in the soil. The volume of the leachates was determined by weighing the leachate collected in the vacuum flask and mineralized N calculated on the basis of dry soil weight.
For determination of soil C mineralization potential after each leaching event, the filter units were placed in sealed mason jars and the headspace swept with CO 2 -free air. Changes in the CO 2 concentration in the head space of the jar due to soil CO 2 evolution was then determined after a period of approximately 45 hours using a gas chromatograph (GC) (Buck Scientific Inc., East Norwalk, CT, USA) fitted with a thermal conductivity detector (TCD).

Statistical Analysis
Data generated in all experimental plots and incubations trials were analyzed by using the SAS statistical program (SAS Institute, 2002-2003. The analysis of variance (ANOVA) was performed by using PROC ANOVA. Means were separated using Fisher's protected least significant difference (LSD) test at the probability level of p ≤ 0.05.

Rainfall Distribution during the Growing Season
The cumulative precipitation across the different field trials during the 2006-2009 growing seasons were similar. From potato planting to harvest during 2006-07 growing season, 365 mm of precipitation was received, during 2007-08, 388 mm was recorded and during 2008-09, 346 mm was registered (Fig. 2). However, the distribution pattern differed among growing seasons with high rainfall concentration in February and March, 2007 and almost an even distribution from the middle of November, 2007 to the middle of March, 2008, and almost uniform distribution from the end of January to end of April 2009. These differences in rainfall distribution among growing seasons may have caused some variability on the effects of application of soil organic and inorganic fertilizers on soil properties.

Initial Soil Analysis
Initial soil chemical and physical characteristics prior to treatment application in the field plots and of organic fertilizers are presented in Table 4. In general, the soils in the field plots initially contained very low amounts of total organic C and total N during both growing seasons and at both elevations, confirming several reports about the low soil quality of agricultural lands of this region (FAO, 1999). In contrast, based on these soil test results, soil test P and K, exchangeable Ca and Mg would not be considered as limiting factors for plant production as reported by Westermann (2005). The lower content of some mineral nutrients in lower elevation communities could possibly be attributed to the relatively greater use of tractor-based tillage, the presence of sandier soils and greater soil erosion caused mainly by wind. For most of the chemical characteristics analyzed, no major differences were found between cow and sheep manure samples either at the higher and lower elevations (Table 5). Both manure types used during the 2006-08 growing seasons had relatively high proportions of total organic C, total N, total K and total P. However, higher concentrations of soil total organic C and total N were observed either in cow and sheep manure at higher elevations (i.e., Kellhuiri and Vinto Coopani) than those at lower elevations (i.e., San Juan Circa and San José de Llanga). Compost had less total organic C than both manure types, and Biofert had higher total organic C and total N, but lower total P and total K than the other organic fertilizers.

Soil chemical properties
Results of selected soil properties analyzed in potato field trials at blooming time during both the 2006-07 and 2007-08 growing seasons and in both higher and lower elevations are presented in Table 6. Soil pH, soil test P, total organic C and total N were not significantly affected by growing season and communities but were significantly affected by application of the organic and inorganic fertilizers. Exchangeable soil total Ca and Mg and the CEC were not affected by any factor. Soil pH was significantly increased with application of cow and sheep manure alone or when either manure was combined with fertilizer. This result confirms research that found increases in soil pH when farm manure (Clark et al., 1998) or organic and inorganic fertilizers were applied to agricultural lands (Mabuhay et al., 2006); although some studies observed very slight soil pH response to addition of either organic or inorganic fertilizers (Nastri et al., 2009), no differences in soil pH receiving organic, inorganic fertilizers or combination of both (Kaur et al., 2005), or even reduction of soil pH when green manure or farmyard manure was used in alkaline soils (Yaduvanshi, 2003).  Table 4. Initial soil properties of the potato field trials in two growing seasons and in four communities of Umala. Samples were collected prior to planting and treatment application.

Values represent the average and ± standard deviation of 4 replicates of soil collected in each community.
Year/ community
Soil test P significantly increased only when inorganic fertilizer alone and combined with either cow or sheep manure were applied. A similar increase of soil available P was observed when inorganic fertilizer, green manure or farmyard manure (Yaduvanshi, 2003), or manure or compost were applied (Eghball, 2004).
Significant interactions were found for the growing season and treatment factors on soil total inorganic N (TIN) (NH 4 + -N and NO 3 --N). In 2006-07 all treatments increased soil TIN, but in 2007-08 only treatments including inorganic fertilizers alone or combined with cow, sheep or both together significantly increased soil TIN. Differences in soil TIN in the control treatment (T1) between the two growing seasons is possibly partially due to the differences in initial soil properties among the new sites used to evaluate the first year