Carbon (C) and organic nitrogen (N) mineralization processes play key roles in supplying nutrient elements essential for plant growth. On the other hand, since the low organic matter content of agricultural soils of arid and semi-arid areas, microbial activities are often limited by carbon in these types of soils. Enhancement of microbial activities and populations could be achieved by returning plant residues to soil. Application of such acid-producing materials as sulfur may eliminate low uptake of some nutrients and slow decomposition of plant residue in calcareous soils, respectively in short and long-term in these soils. Therefore, the purpose of this study was to evaluate the interactive effects of plant residue and sulfur additions on microbial activities including C and N mineralization as well as their mineralization rates in a calcareous soil from Iran. This study was performed using a factorial experiment arranged in a Completely Randomized Design (CRD) with three replications under laboratory incubation conditions. Treatments included were different plant residues (i.e., wheat, alfalfa, corn, rice, almond, walnut as well as grape) and sulfur levels (i.e. 0, 5 and 10 t ha-1). We evaluated effects of plant residue and sulfur applications on C and N mineralization to over 97 days of incubation period. Results show that plant residue application had a positive and significant effect on carbon and nitrogen mineralization and their mineralization rates, and led to C mineralization enhancement up to two fold. The effect of plant residue on N mineralization depends on the chemical quality and C/N ratio of the residue. The effect of sulfur additions on C mineralization and its mineralization rate was highly significant, as it caused an increase in microbial respiration during incubation time but its addition didn’t have significant effect on N mineralization and its mineralization rate. The interactive effects of plant residue and sulfur utilization on C and N mineralization primarily depend on chemical quality of plant residues and the level of applied sulfur. Briefly, the residues with more recalcitrant compounds such as lignin, showed less microbial activities.

Iran as an arid and semiarid country has been faced with low precipitation and water shortage. In this regard, the application of suitable management practices using plant residues is one of the most important strategies resulting in the improvement of organic matter content as well as water retention of soils. Therefore, in many soils of arid and semiarid regions, organic matter is as the best amendment to increase water content and improve physical properties of the soil (23, 40). Plant residue can increase soil moisture content through the improvement of soil organic carbon content and the reduction of runoff and evaporation (9, 25). Application of plant residue is one of the common methods for improving soil properties. However in some areas, plant residues are removed from the filed or burned by the farmers. This study was performed to investigate the effect of different types of plant residues and their management on soil moisture content as well as aeration properties under field conditions.
The experiment was conducted under field conditions as factorial statistical design based on RCBD consisting three replicates. The first factor was residue type including barely straw and alfalfa residue and the second factor was different managements of the residues including 1) incorporating one percent of the plant residues with the soil, 2) incorporating 0.5 percent of the plant residues with the soil, 3) surface retention of plant residues, 4) burning of plant residues and 5) control. After nine months, the final soil moisture content at seven suctions including 0, 30, 50, 100, 500, 1000 and 1500 kPa was measured. In addition, the available water content and aeration porosity were calculated.
The result showed that among the experimental treatments, the incorporation and also surface retention of plant residues increased soil moisture compared to control, whereas, burning the plant residues reduced the moisture content. In comparison to the control, the incorporation of one percent barely straw to the soil led to the highest increases in the moisture content at PWP (70%) and the available water (12.5%). Reversely, burning of barely straw showed the highest reduction in the moisture content at PWP (34.5%) compared to control. In addition, the surface retention of plant residues resulted in the highest increase (48%) in aeration porosity compared to control.
The findings of this study showed the favorite efficiency of incorporating and surface application of organic sources in improving soil moisture and aeration porosity.

In order to investigate the yield and components of safflower under plant residue treatments, nitrogen and micro element in Ahvaz, this experiment was conducted during 2010-2011 cropping season in Agriculture College of Shahid Chamran University of Ahvaz by using split plot based on Randomized Complete blocks design with three replications. Different values of plant residues at four levels (complete residues (5 ton/ha straw), incomplete plant residues (3 ton/ha straw), burning of plant residues and elimination of the plant residues) and sub-factor different values of nitrogen fertilizer at 3 levels (consumption of 150, 100 and 50 kg N/ha) and sub-sub-factor micro elements of spray at 2 levels (micro elements unsparing and spraying recommendeddose of fertilizer) was considered. Based on the results, a significant difference in grain yield, captiual per plant, grain per captiual, 1000 grain weight, and oil percentage was showed. Maximum grain yield resulted from treatment of incomplete plant residues, 100 kg N/ha and spray of micro elements (with mean 6278.87 kg/h). Minimum grain yield resulted from burning of plant residues, 50 kg/h and micro elements (with mean 1771.48 kg/h). The highest percentage of oil was equal to 33.5% (in complete plant residues with 150 kg N/ha and spraying micro elements) but that was not significant from 31% that was obtained in incomplete plant residues with 100 kg N/ha with spraying micro elements.

Application of different organic materials in organic agriculture may change the status of plant nutrients in deficient soils. Arid soils of Iran may have a considerable content of potassium (K); but their K content is decreasing due to intensive agriculture. This K deficiency may be alleviated by different organic materials application in organic agriculture.
In the current investigation, a completely randomized experiment was done with application of four plant residues and their produced biochars to a calcareous soil and their effect on different forms of K. Three grams of wheat straw, corn straw, rice bran and licorice root residue and their produced biochars was added to 100 g of a clay loam calcareous soil and incubated for 90 days at 22±2 °C and 50 % of saturation moisture content. The soil samples were air-dried and sieved and pH, electrical conductivity and contents of soluble, exchangeable, non-exchangeable, HNO3-extractable K and K release rate from soil minerals were determined.
Results indicated that plant residues had no effect on soil pH, but all biochars increased soil pH (mean of 0.07). Soil EC was increased with application of wheat and corn straws and conversion of plant residues to biochars had more effect on soil salinity. Licorice root residue and its biochar had no effect on the content of different K forms; but other plant residues and their biochars increased soluble, exchangeable and HNO3-extractable K in the order of wheat residue > corn residue > rice bran. On average, biochars had more effect than plant residues on the content of soluble, exchangeable and HNO3-extractable K (212, 269 and 286 mg kg-1, respectively). The content of HNO3-extractable K was not affected with plant residues and their biochars (except for corn straw). Wheat and corn straws and rice bran released 286, 217 and 146 mg K kg-1, respectively; and their biochars released 637, 429 and 290 mg K kg-1, respectively from K-bearing minerals and this may be due to the effect of organic molecules and non-organic cations of organic materials on mineral weathering and K release.
It is concluded that application of plant residues and their biochars may have significant effects on soil K status and alleviation of K deficiency and the role of biochar is more important than primary plant residues. On the other hand, increase in soil salinity and pH especially in calcareous soils of arid land should be take into consideration.

Improvement of soil properties through sustainable agriculture requires suitable management of plant residues. This study was conducted to investigate the effect of different types and managements of plant residue on soil physical properties in a field experiment as factorial based on RCBD with three replicates. The first factor was residue type including barley straw and alfalfa residue، and the second was different managements including 1) incorporating one percent of the plant residues with the soil (MIX1)، 2) incorporating 0. 5 percent of the plant residues with the soil (MIX0. 5)، 3) surface retention­­ of plant­ residues (SUR)، 4)، burning plant residues (BUR) and 5) control (C). The results showed that the applied managements of plant residue had significant effects on the selected soil properties. In comparison to the control، MIX1 application of barley straw resulted in the highest increases in organic carbon content (70%)، total porosity (23%) and final infiltration rate (82%)، and the highest reduction in bulk density (40%). In contrast، burning the alfalfa residue led to the highest reductions in organic carbon (200%)، total porosity (29. 5%)، the final infiltration rate (30%)، and the highest increase in bulk density (5. 5%). In addition، MIX0. 5 and SUR treatments improved the soil properties significantly. The highest and the least infiltration rates were found in MIX1 of barley straw (48. 6 mm/h) and BUR of alfalfa residue (4. 6 mm/h)، respectively. Overall، with respect to the improvement of soil properties، the managements of plant residue were prioritized as: MIX1>MIX0. 5>SUR>C>BUR.

This research was conducted in order to study the effect of using plant residues and zinc sulfate on the absorbability of zinc in soil and its concentration in the grain wheat as well as the effect on the concentration of phytic acid, phytic acid molar ratio and grain protein. The experiment was conducted in the form of a statistical randomized complete blocks design with eight treatments (control, zinc sulfate, alfalfa plant residues, beans and wheat and alfalfa plant residues, beans and wheat plus zinc sulfate) in three replications in a farm in Isfahan province, Dehaghan city. Due to the amount of organic matter and zinc in the soil, zinc sulfate by 60 kilogram per hectare and plant residues by 10 tons per hectare was added to the considered treatments. After that, the amount of 250 kilogram per hectare of common grain wheat of region (Sepahan cultivar) was planted. The obtained results showed that the application of zinc sulfate in the soil on the yield grain and zinc concentration of grin was significant. In treatment that plant residues with zinc sulfate was added in the soil, there was a significant increase in the measured properties compared to the control and plant residues application and zinc sulfate treatments. Among the plant residues of wheat, beans and alfalfa, alfalfa residues led to more increase in zinc concentration and grain protein and more decrease phytic acid and phytic acid to zinc molar ratio in grain. It can be concluded that the application of zinc with plant residues can cause more increasing in zinc concentration and grain qualitative parameters.

Nowadays, environmental aspects of soil erosion are specifically considered by researchers. In this regard, secondary (aggregate) particle size distribution (SPSD) is so important in different issues such as nutrient loss, pollutant transport and carbon cycle. Aggregate size distribution is known as the representative of soil structure, which is by itself regarded as a dynamic soil feature. The type and application method of plant residues have a significant influence on the aggregate size distribution. Therefore, the accurate utilization of plant residue management after the harvesting procedure is of crucial environmental issues. However, farmers in some parts of the country, burn plant residues remaining after harvesting; this leads to the destruction of soil structure, greenhouse gas emissions and environmental damages. Therefore, the purpose of the current study was to investigate the effects of the type and different managements of plant residues on the aggregate size distribution indices.
THEORETICAL FRAMEWORK: Several studies around the world and in Iran have been conducted on the impact of different management methods of crop residues on the aggregate size distribution. In this context, it has been found that through mixing and/or keeping crop residue after harvesting, the mean weight diameter (MWD) of the aggregates increased significantly in both wet and dry cases as compared with the control. On the other hand, due to the positive effects of these two methods of management on the structural stability compared to burning, the formation and development of the surface crust decreases. Compared to the favorable effects of mixing and keeping plant residues in farm on aggregate size distribution, some destructive operations such as burning have negative influences.
To obtain the objectives, a field experiment was done as factorial based on RCBD with three replicates. The first factor was the residue type including barely straw and alfalfa residue, and the second factor was different management practices including 1) incorporating one percent of the plant residues into the soil, 2) incorporating 0.5 percent of the plant residues into the soil, 3) surface retention­­ of plant­ residues, 4) burning of plant residues and 5) control. After nine months, the aggregate size distribution indices including the mean weight diameter (MWD) and the geometric mean diameter (GMD) both at two cases of wet and dry and also water stable aggregates (WSA) and dry stable aggregates (DSA) were measured.
Results of the variance analysis showed that the influence of plant residue type was just significant on MWD and GMD in wet case, whereas, the effect of plant residue management was significant on all the studied indices. This indicates that the management of plant residues is more important than the type of residues. Also, barley straw was more effective than alfalfa residue in improving the aggregate size indices, which was attributed to higher content of carbon and greater C:N ratio in the composition of barley straw. Also, in all cases, mixing one percent of plant residues with soil showed greater role in improving the soil structure than the other application methods of residues. The incorporation of one percent barely straw led to the highest influence on the improvement of aggregate size distribution, so that due to the application of this treatment as compared with control, MWD and GMD increased respectively by 87% and 68.6% (under wet conditions) and by 33.6% and 21% (under dry conditions), while WSA and DSA were enhanced by 86.4% and 23.7%, respectively. Moreover, the surface application of residues resulted in aggregate size improvement, however, its effectiveness was less than the incorporation method. In contrast, the burning of residues reduced almost all the indices compared to the control.
CONCLUSIONS & SUGGESTIONS: In general, the priority of residue application methods regarding the improvement of aggregate size distribution was determined respectively as: burning.

The incorporation of plant residues into soil is a recommended management practice for maintaining or increasing the level of soil organic matter. Although, its effect may be controlled by the soil type, climate and the initial quality of the plant residues. On the other hand, climate on a global scale is changing and the increase of drought stress is predicted in a large scale. As a result of these changes, all soil processes that are related to soil moisture and temperature, including mineralization of nutrients are being affected. This study was conducted to investigate the interactive effects of drought stress and plant residues quality on N mineralization in soils treated with different plant residues.
For this purpose, four types of plant residues including alfalfa, clover, corn and wheat shoots were collected. Plant residues-treated soils along with a control (without plant residue application) were incubated under three soil moisture levels 25, 35 and 55% of water holding capacity at 25৹C for 150 days. At the end of incubation period, net nitrification, net ammonification, net nitrogen mineralization and net production of soluble organic nitrogen were measured.
The results indicated that the maximum amount of net nitrification (282.33 mg NO3- kg-1), net ammonification (52.11 mg NH4 kg-1), net nitrogen mineralization (334.44 mg NO3-㑾 kg-1) and net changes of soluble organic nitrogen (141.43 mg N kg-1) were observed in the alfalfa-treated soils. The measured indices were significantly increased as the level of soil water during the incubation increased. Net nitrification (r=0.798***), net nitrogen mineralization (r=0.816***) and net changes of soluble organic nitrogen (r=0.803***) had a positive and significant correlation with nitrogen concentration of the plant residues. Negative significant correlations were observed between the measured N transformation indices and lignin content (r= -0.589***, r= -0.605*** and r= -0.612***) and ratios of C/N (r= -0.418***, r= -0.410*** and r= -0.424***) and LG/N (r= -0.650***, r= -0.655*** and r= -0.657***), respectively. Net ammonification did not significantly correlated with net nitrification (r=-0.007), net nitrogen mineralization (r=0.051) and net changes of soluble organic nitrogen (r=0.048). Under drought stress conditions, net nitrification (r=.998***) was more strongly correlated with net nitrogen mineralization than net ammonification (r=0.051).
In conclusion, the effect of drought stress on the N transformation indices were not similarly observed among the plant residue-treated soils. The soils treated with plant residues with greater N content (alfalfa and clover) showed to be more negatively affected by the drought stress compared to those treatments contained less initial N or greater C:N or lignin:N ratios (corn and wheat).

Allelopathic effects of plant residues is an important research avenue regarding optimization of rotation systems in agronomy. The aim of this study was to investigate the allelopathic effects of four plant residues, namely, black cumin, dragonhead, dill, and soybean on the germination and growth of wheat (Triticum aestivum) in different cropping systems. Results showed that application of organic manure for previous crops reduced the residue phytotoxicity and consequently alleviated the adverse effect of plant residues on the leaf area, length, and dry weight of the wheat root, affecting chlorophyll a, chlorophyll b, and carotenoids of wheat seedling leaves. In the presence of plant residue, the length and dry weight of the wheat roots were more negatively affected in comparison with shoots. The greatest allelopathic inhibition was observed for the wheat cultivated in the residue of black cumin, but soybean, dill, and dragonhead residues also potentially showed inhibition effects. It can be concluded that agroecosystems in which autumn wheat is in the rotation should be avoided where there are residues of soybean, black cumin, dragonhead, and dill. The tillage system in the same condition may not be agronomically suitable because of the allelopathic effects of previous crops.

Soil microbial activity and biochemical processes are often limited by carbon availability in arid- and semi-arid regions, probably due to the low organic matter content. Consequently, return of plant residues to soil is a convenient and effortless practice for increasing microbial activities and biochemical reactions. The primary objective of this study was to evaluate the effect of various plant residues on soil microbial respiration and biomass, and enzymatic activities as well. The experiment consisted of a completely randomized design (CRD) with three replications under laboratory conditions. Experimental treatments consisted of seven plant residues including wheat, alfalfa, corn, rice, almond, walnut and grape, common in agro-ecosystems of Chaharmahal va Bakhtiari province, with a control soil without plant residue addition. Results show that the added plant residues brought about a significant increase in microbial activity (soil respiration) and biomass with concurrent increases in enzyme activities in the studied soil. The results of the current study indicate that enzyme activities would alter with changes in substrate quality during the course of plant residue decomposition. However, the extent to which, soil microbial activity and biomass, and enzyme activities fluctuate depended largely upon the type and quality of plant residues used, and the stage of residue decomposition.