Nginx rewrite rules for subdirectory

Question

I recently moved to nginx and now I have several problems. On main domain "domain.tld" i run Wordpress that has a lot of rewrite rules for non existent files (for clean-seo urls). In a sub-directory "domain.tld/qa/" i have Question2Answer engine, that also needs clean urls, but I can't seem to get them working.

Here are the rewrite rules that I use on main domain.

    server {

listen   80; 

server_name domain.tld;
        root        /var/www/domain.tld;
        access_log  /var/log/nginx/access.log;
    error_log  /var/log/nginx/error.log;
index index.php index.html index.htm;

# BEGIN W3TC Page Cache cache
location ~ /wp-content/w3tc/pgcache.*html$ {
    expires modified 3600s;
    add_header Vary "Accept-Encoding, Cookie";
    add_header Pragma "public";
    add_header Cache-Control "max-age=3600, public, must-revalidate, proxy-revalidate";
}
location ~ /wp-content/w3tc/pgcache.*gzip$ {
    gzip off;
    types {}
    default_type text/html;
    expires modified 3600s;
    add_header Vary "Accept-Encoding, Cookie";
    add_header Pragma "public";
    add_header Cache-Control "max-age=3600, public, must-revalidate, proxy-revalidate";
    add_header Content-Encoding gzip;
}
# END W3TC Page Cache cache
# BEGIN W3TC Browser Cache
gzip on;
gzip_types text/css application/x-javascript text/richtext image/svg+xml text/plain text/xsd text/xsl text/xml image/x-icon;
location ~ \.(css|js)$ {
    expires 31536000s;
    add_header Pragma "public";
    add_header Cache-Control "max-age=31536000, public, must-revalidate, proxy-revalidate";
}
location ~ \.(html|htm|rtf|rtx|svg|svgz|txt|xsd|xsl|xml)$ {
    expires 3600s;
    add_header Pragma "public";
    add_header Cache-Control "max-age=3600, public, must-revalidate, proxy-revalidate";
}
location ~ \.(asf|asx|wax|wmv|wmx|avi|bmp|class|divx|doc|docx|exe|gif|gz|gzip|ico|jpg|jpeg|jpe|mdb|mid|midi|mov|qt|mp3|m4a|mp4|m4v|mpeg|mpg|mpe|mpp|odb|odc|odf|odg|odp|ods|odt|ogg|pdf|png|pot|pps|ppt|pptx|ra|ram|swf|tar|tif|tiff|wav|wma|wri|xla|xls|xlsx|xlt|xlw|zip)$ {
    expires 31536000s;
    add_header Pragma "public";
    add_header Cache-Control "max-age=31536000, public, must-revalidate, proxy-revalidate";
}
# END W3TC Browser Cache
# BEGIN W3TC Page Cache core
rewrite ^(.*\/)?w3tc_rewrite_test$ $1?w3tc_rewrite_test=1 last;
set $w3tc_rewrite 1;
if ($request_method = POST) {
    set $w3tc_rewrite 0;
}
if ($query_string != "") {
    set $w3tc_rewrite 0;
}
set $w3tc_rewrite2 1;
if ($request_uri !~ \/$) {
    set $w3tc_rewrite2 0;
}
if ($request_uri ~* "(sitemap\.xml(\.gz)?)") {
    set $w3tc_rewrite2 1;
}
if ($w3tc_rewrite2 != 1) {
    set $w3tc_rewrite 0;
}
set $w3tc_rewrite3 1;
if ($request_uri ~* "(\/wp-admin\/|\/xmlrpc.php|\/wp-(app|cron|login|register|mail)\.php|wp-.*\.php|index\.php)") {
    set $w3tc_rewrite3 0;
}
if ($request_uri ~* "(wp\-comments\-popup\.php|wp\-links\-opml\.php|wp\-locations\.php)") {
    set $w3tc_rewrite3 1;
}
if ($w3tc_rewrite3 != 1) {
    set $w3tc_rewrite 0;
}
if ($http_cookie ~* "(comment_author|wp\-postpass|wordpress_\[a\-f0\-9\]\+|wordpress_logged_in)") {
    set $w3tc_rewrite 0;
}
if ($http_user_agent ~* "(W3\ Total\ Cache/0\.9\.2\.3)") {
    set $w3tc_rewrite 0;
}
set $w3tc_ua "";
if ($http_user_agent ~* "(2\.0\ mmp|240x320|alcatel|amoi|asus|au\-mic|audiovox|avantgo|benq|bird|blackberry|blazer|cdm|cellphone|danger|ddipocket|docomo|dopod|elaine/3\.0|ericsson|eudoraweb|fly|haier|hiptop|hp\.ipaq|htc|huawei|i\-mobile|iemobile|j\-phone|kddi|konka|kwc|kyocera/wx310k|lenovo|lg|lg/u990|lge\ vx|midp|midp\-2\.0|mmef20|mmp|mobilephone|mot\-v|motorola|netfront|newgen|newt|nintendo\ ds|nintendo\ wii|nitro|nokia|novarra|o2|openweb|opera\ mobi|opera\.mobi|palm|panasonic|pantech|pdxgw|pg|philips|phone|playstation\ portable|portalmmm|\bppc\b|proxinet|psp|qtek|sagem|samsung|sanyo|sch|sec|sendo|sgh|sharp|sharp\-tq\-gx10|small|smartphone|softbank|sonyericsson|sph|symbian|symbian\ os|symbianos|toshiba|treo|ts21i\-10|up\.browser|up\.link|uts|vertu|vodafone|wap|willcome|windows\ ce|windows\.ce|winwap|xda|zte)") {
    set $w3tc_ua _low;
}
if ($http_user_agent ~* "(acer\ s100|android|archos5|blackberry9500|blackberry9530|blackberry9550|blackberry\ 9800|cupcake|docomo\ ht\-03a|dream|htc\ hero|htc\ magic|htc_dream|htc_magic|incognito|ipad|iphone|ipod|kindle|lg\-gw620|liquid\ build|maemo|mot\-mb200|mot\-mb300|nexus\ one|opera\ mini|samsung\-s8000|series60.*webkit|series60/5\.0|sonyericssone10|sonyericssonu20|sonyericssonx10|t\-mobile\ mytouch\ 3g|t\-mobile\ opal|tattoo|webmate|webos)") {
    set $w3tc_ua _high;
}
set $w3tc_ref "";
if ($http_cookie ~* "w3tc_referrer=.*(google\.com|yahoo\.com|bing\.com|ask\.com|msn\.com)") {
    set $w3tc_ref _search_engines;
}
set $w3tc_ssl "";
if ($scheme = https) {
    set $w3tc_ssl _ssl;
}
set $w3tc_enc "";
if ($http_accept_encoding ~ gzip) {
    set $w3tc_enc .gzip;
}
if (!-f "$document_root/wp-content/w3tc/pgcache/$request_uri/_index$w3tc_ua$w3tc_ref$w3tc_ssl.html$w3tc_enc") {
    set $w3tc_rewrite 0;
}
if ($w3tc_rewrite = 1) {
    rewrite .* "/wp-content/w3tc/pgcache/$request_uri/_index$w3tc_ua$w3tc_ref$w3tc_ssl.html$w3tc_enc" last;
}

# END W3TC Page Cache core

    # enforce NO www
     if ($host ~* ^www\.(.*))
    {
        set $host_without_www $1;
        rewrite ^/(.*)$ $scheme://$host_without_www/$1 permanent;
    }

    # unless the request is for a valid file, send to bootstrap
    if (!-e $request_filename)
    {
        rewrite ^(.+)$ /index.php?q=$1 last;
    }

    # catch all
    error_page 404 /index.php;

    # use fastcgi for all php files
    location ~ \.php$
    {
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }

    # deny access to apache .htaccess files
    location ~ /\.ht
    {
        deny all;
    } 



}

I tried adding

if (!-f $request_filename)
    {
        rewrite ^/qa/(.*)$ /qa/index.php?qa-rewrite=$1 last;

    }

and

location /qa {
     try_files /qa/$uri $uri/ /index.php?qa-rewrite=$uri$args;
} 

but that didn't help. The Apache .htaccess file for Question2Answer engine looks like this:

DirectoryIndex index.php
RewriteEngine On
#RewriteBase /your-sub-directory
RewriteCond %{REQUEST_URI} ^(.*)//(.*)$
RewriteRule . %1/%2 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^.*$ index.php?qa-rewrite=$0&%{QUERY_STRING} [L]

Can anyone please help?

I'm sorry if this is a stupid question, i just don't know much about nginx rewrite rules yet.

Thank you.