The productivity of rainfed farming is also affected by the selection of planting date, which often depends on the timing of the first effective rainfall events. For this joint soil-terrain-climate analysis, all regions with a growing season of two months or shorter were assigned a suitability value of zero and thus classified as unsuitable for agriculture. We then evaluated the capacity of land for rainfed farming by using a precipitation cut-of of 250 mm year−1 , which is often regarded as the minimum threshold for the rainfed farming . As shown in Table 1, the inclusion of the length of growing period and precipitation threshold into the analysis only slightly reduced the total area of high-quality lands from 5.8 to 5.4 million ha. This implies that most lands with suitable soil and terrain conditions also receive sufficient amount of moisture to sustain rainfed agriculture. On the contrary, the area of unsuitable lands increased from 39.7 to 112.9 million ha when precipitation and duration of growing season thresholds were superimposed on the soil and topographic constraints. This increase in unsuitable acreage was mainly driven by the demotion of lands from the very poor class to the unsuitable class . The addition of moisture constraints also reduced the area of medium suitability lands by 4.8 million ha. In summary, for the rainfed farming suitability analysis, 125 million ha of Iran’s land might be classified as poor or lower ranks whilst only 18 million ha meet the required conditions for the medium or higher suitability classes . The geographical distribution of these land classes is mapped in Fig. 4. Almost the entire central Iran ,growing strawberries vertical system and the vast majority of land area in the eastern , southeastern and southern provinces were found to be unsuitable for rainfed farming. Almost half the area of Khuzestan and three-quarters of Fars provinces were also characterized unsuitable. Over the entire east, only in the northern part of Khorasan Razavi province, is there a belt of marginally suitable lands satisfying the requirements of a potentially prosperous rainfed agriculture .
In the next step of the analysis, the suitability of land was scaled with the annual precipitation over the range of 100 to 500 mm year−1 . The lower limit is deemed to exclude the desert areas for agricultural use whilst the upper limit represents a benign moisture environment for the growth of many crops. This last analysis, here after referred to as precipitation scaling method, makes no assumption as to whether the cropping practices rely on rainfall or irrigation to satisfy crop water requirement and may thus represent a more comprehensive approach for agricultural suitability assessment. The same minimum length of growing period and soil/topographic constraints as with the two previous methods were used in this analysis. Compared to the rainfed agriculture analysis, the precipitation scaling method mainly changed the distribution of lands within the lower suitability classes . For example, a great proportion of lands within the unsuitable class was shifted up to the very poor and poor classes. This implies that, to a limited extent, irrigation can compensate for the below threshold precipitation . Nevertheless, water availability cannot necessarily justify agriculture in areas with low soil and topographic suitability. This has an important implication for water management in Iran that has a proven record of strong desire for making water available to drier areas through groundwater pumping, water transfer, and dam construction. The majority of high-quality lands , which also retains sufficient levels of moisture are located in the western and northern provinces of Iran . Kermanshah province accommodates the largest area of such lands followed by Kurdistan . High-quality lands were estimated to cover 33% and 21% of these two provinces, respectively. Other provinces with high percentages of high quality lands were Gilan , Mazandaran , West Azerbaijan , and Lorestan . For 17 provinces, however, high-quality lands covered less than 1% of their total area .To estimate the total area of croplands within each suitability class, we visually inspected 1.2 million ha of Iran’s land by randomly sampling images from Google Earth . The proportion of land used for cropping increased almost linearly with the suitability values obtained from the precipitation scaling method . Total cropping area in Iran was estimated to be about 24.6 million ha, which is greater than the reported value by the Iran’s Ministry of Agriculture. This authority reports the harvested area; hence, the fallow or abandoned lands are not included in their calculation of active agricultural area.
Our visual method, however, captures all lands that are currently under cultivation or had been used for cropping in the near past that are now in fallow or set-aside . The relative distribution of croplands amongst the suitability classes shows that about 52% of the croplands in Iran are located in areas with poor suitability or lower ranks as identified by the precipitation scaling method. Particularly concerning are the 4.2 million ha of lands that fall within the unsuitable class. Approximately 3.4 million ha of cropping areas occur in good and very good lands . However, no agricultural expansion can be practiced in these areas as all available lands in these suitability classes have already been fully exploited. Medium quality lands comprise 12.8 million ha of Iran’s land surface area , of which about 8.6 million ha have been already allocated to agriculture . Nevertheless, due to their sparse spatial distribution and lack of proper access, only a small portion of the unused lands with medium suitability can be practically deployed for agriculture. Using FAO’s spatial data on rainfed wheat yield in Iran, we estimated the mean yield for wheat cropping areas located within each of the six suitability classes. As shown in Fig. 7, the yield of the rainfed wheat increased proportionally with improving suitability index, showing that our suitability index adequately translates to crop yield. Using the observed yield-suitability relationship , we estimated that 0.8 million ton of wheat grain might be produced per year by allocating 1 million ha of the unused lands from the medium suitability class to rainfed wheat cropping.Whilst the insufciency of water resources has long been realized as a major impediment to developing a productive agriculture in Iran, our study highlights the additional limitations caused by the paucity of suitable land resources.Tat is, Iran as a member of Convention on Biological Diversity is obliged to fulfil Aichi Biodiversity Targets whose Target 11 requires Iran to expand its protected area to 17% by 2020, which is almost double the size of the current protected areas in Iran .
Agriculture also needs to compete with other types of land uses with urbanization being an important driver of agricultural land loss. By converting arable lands to a barren desert, desertification is a growing global concern, particularly in the MENA region and Iran. The redistribution of croplands from the low-quality lands to more suitable ones has potentials to improve crop yields and the sustainability of agriculture in Iran. A recent global-scale study concluded that by reallocating croplands to suitable environmental conditions, the global biomass production could increase by 30% even without any land expansion. However, reallocation planning requires accurate mapping of croplands, which is not currently available for Iran. Inefficient agricultural practices in unsuitable lands need to be avoided as they produce little yields at the cost of exacerbating land degradation and water scarcity problem. Our estimations shows that rainfed wheat production from a small acreage of 1.0 million ha in the medium suitability class can equal that from 5.5 million ha of lands in unsuitable or very poor areas . Although this conclusion may not hold for other crops grown in Iran,growing vegetables in vertical pvc pipe the wheat crop could be a good candidate to make such a generalization as wheat is the most widely cultivated crop in the country and is considered as a very low demanding plant, which has adapted to a broad range of contrasting environments. Redistribution of croplands, however, will not be a trivial task for both the Iranian decision makers and stakeholders due to various sociol-economic and logistic barriers. Lands found suitable for agriculture may not be easily accessible if scattered sparsely or occur in remote areas. Given the land and water limitations, increasing the crop production in Iran needs to be achieved through sustainable intensification, which has been found a promising approach for ensuring food security in several global-scale studies. As such, it is of vital importance for Iran to properly use its limited agricultural lands, improve water use efficiency, optimize crop pattern distribution, and adopt modern cultivation techniques. Practicing certain industrial agriculture methods in the unsuitable lands might be a viable strategy to sustainably maintain these lands in the agricultural sector while avoiding the potential socio-economic and political costs associated with redistribution of agricultural lands and farming populations. For example, protected agriculture can be established at some of these locations to cope with both land suitability and water availability constraints. While water insufficiency is a major limiting factor for both field and protected farming, the latter will be affected to a lesser extent. Our suitability assessment is based on a general set of requirements known to affect the productivity of a large number of crops, but there would exist crops with exceptional adaptive traits that can grow under less favourable conditions.
Although we used the most updated geospatial data at the finest available resolution, the result of our suitability analysis should be interpreted in commensuration with the reliability and quality of the original data. For example, whereas the GlobCover database reliably maps the distribution of forests and rangelands in Iran, our visual inspection of satellite images showed that sometimes their utilized method lacks the required precision to distinguish cultivated from uncultivated croplands. Although soil erosion was not directly incorporated into our analysis, the use of slope at the very high resolution implicitly accounts for this effect. The interaction between variables and the quality of subsoil are among other factors that can be considered in the future studies. This study used precipitation as the only water availability factor. Including surface water and groundwater availability can further improve the adequacy of the land evaluation analysis. Given the good correlation between water availability and land suitability for agriculture, the general findings of this study are not expected to change significantly by the inclusion of water availability conditions. Nevertheless, due to the current water shortage constraints across the country, the potential agricultural capacity of the country is likely to decrease when water availability is added to the analysis. Although global projections suggest that the suitable lands may expand with climate changes, how these changes, particularly in precipitation pattern, would affect the suitability of Iran’s land for crop production in the future is subject to high degree of uncertainty and needs further work.Atmospheric carbon dioxide has increased about 35% since 1800 , and computer models predict that it will reach between 530 and 970 ppm by the end of the century . This rise in carbon dioxide could potentially be mitigated by crop plants, in which photosynthesis converts atmospheric carbon dioxide into carbohydrates and other organic compounds. The extent of this mitigation remains uncertain, however, due to the complex relationship between carbon and nitrogen metabolism in plants . Carbon metabolism provides the energy and carbon molecules to synthesize organic nitrogen compounds in plants, whereas nitrogen metabolism provides the amino groups for all proteins . Proteins include all enzymes that catalyze biochemical reactions in plants, including carbon metabolism. Any environmental perturbation that interferes with nitrogen metabolism sooner or later inhibits carbon metabolism.The focal point of crop responses to rising carbon dioxide levels is the enzyme rubisco . Rubisco is the most prevalent protein on Earth and contains as much as half of the nitrogen in plant leaves. It catalyzes two different chemical reactions: one reaction combines a 5-carbon sugar RuBP with carbon dioxide, and the other reaction combines this same sugar with oxygen. The reaction of RuBP with carbon dioxide produces a 6-carbon compound that immediately divides into two molecules of a 3-carbon compound , hence the name C3 carbon fi xation .